It has been asserted that our subject had a German origin (viz., SELLER/ZELLER/KELLER), but he was almost certainly Scottish or English (please see my comments among the Sources below and on this page). 

1.  LDS.  Family Search: Internet Genealogy Service: IGI - International Genealogical Index (online at FamilySearch.org):  information here is entirely secondary, so caveat emptor!

Thomas CELLAR Spouse: Martha MC COY Marriage: Abt. 1770, Mercersburg, Franklin, Pennsylvania Source: patron submission

Thomas SELLARS Birth: Abt. 1750, Of, Greene, Pennsylvania Spouse: Sarah FLENNIKEN Source: none given

Thomas CELLER Spouse: Sarah FLENNIKEN Marriage: Abt. 1783, Of, Prince Georges, Maryland Source:  Fiche #6142813 [but no matching title found in catalog]

Margaret CELLAR Christening: 7 Apr 1771, Mercersburg, Franklin, Pennsylvania Father: Thomas CELLAR; Mother: Martha MC COY Source: patron submission

Jane CELLAR Christening: 11 Jul 1773, Mercersburg, Franklin, Pennsylvania Father: Thomas CELLAR; Mother: Martha MC COY Source: patron submission

Hannah CELLAR Birth: 19 Feb 1776, Of, Mercersburg, Franklin, Pennsylvania Father: Thomas CELLAR; Mother: Martha MC COY Source: patron submission

Thomas CELLAR Birth: 19 Jan 1784, Franklin, Pennsylvania Father: Thomas KELLER; Mother: Sarah FLANNAGAN Source: patron submission

Robert Mccoy CELLAR Birth: 20 Nov 1785, Franklin, Pennsylvania Father: Thomas KELLER; Mother: Sarah FLANNAGAN Source: patron submission

John Flanagan CELLAR Birth: 1 Mar 1788, Franklin, Pennsylvania Father: Thomas KELLER; Mother: FLANNAGAN Source: patron submission

James CELLAR Birth: 26 Mar 1795, Franklin, Pennsylvania Father: Thomas KELLER; Mother: FLANNAGAN Source: patron submission

Joseph CELLAR Birth: 29 Sep 1798, Franklin, Pennsylvania Father: Thomas KELLER; Mother: FLANNAGAN Source: patron submission

2.  Census Index:  Colonial America, 1607-1789 (Broderbund CD-310):

1778 Fidelity Oath

Cellar, George

MD

Washington Co.

George was the only CELLAR (or variation) in MD or PA.  There were many more CELLARs in the south, including several Thomas'es.

3.  1790 Census Images (online at Ancestry.com; Image #9 of 15; extracted by Diana Gale Matthiesen):

1790

PA

Franklin Co.

[mixed twps]

p. 309

Thomas Cellars

2-3-4-0-1

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
2 free white males	16 or over	in or bef. 1774	= Thomas = Son A
3 free white males	15 or under	1774-1790	= Thomas (b. 1784) = Robert (b. 1785) = John (b. 1788)
4 free white females	all ages	in or bef. 1790	= Sarah (b. 1754) = Margaret (b. 1771) = Jane (b. 1773) = Hannah (b. 1776)
1 slave	all ages	in or bef. 1790	= name unknown

It's very unusual for someone in Pennsylvania to own a slave, but not so for someone in Maryland, or recently from Maryland — and Franklin Co., PA, is just across the state line from Washington Co., MD.  Thomas is the only CELLAR (or variation) on the entire CD.  Listed five lines from Andrew FLANEGAN.

4.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312):  there was no 1800 census in Ohio (it did not become a state until 1803); where available, tax lists are substituted on this CD:•

1806 Tax List	Cellars, Thomas	OH	Franklin [now Delaware] Co.		p. 2
1807 Tax List	Cellars, Thomas	OH	Franklin [now Delaware] Co.		p. 4
1808 Tax List	Cellars, Thomas	OH	Franklin [now Delaware] Co.	Mixed Twps.	p. 5

5.  Census Index:  U.S. Selected Counties, 1810 (Broderbund CD-313):  the entire 1810 Ohio census was lost (except for Washington Co.); where available, tax lists are substituted on the CD:•

1810 Tax List

Cellar, Thomas

OH

Delaware Co.

p. 5

6.  1820 Census Index/Images (online at Ancestry.com; Image #1 of 3; extracted by Diana Gale Matthiesen):¤•

1820

OH

Delaware Co.

Liberty Twp.

p. 93A

Ln. 3

Sarah Cellar

011320-10021-0-500

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	10-15	1804-1810	= Betty's son?
1 male	16-18	1801-1804	= Betty's son?
2 males	19-25*	1794-1801	= Joseph (b. 1798) = James (b. 1795)
2 males	26-44	1775-1794	= George (b. 1791) = Robert (b. 1785)
1 female	9 or under	1810-1820	= George's daughter
2 females	26-44	1775-1794	= George's wife? = Betty FISHER?
1 female	45 or over	in or bef. 1775	= Sarah (b. 1754)
5	persons engaged in agriculture
*In the 1820 Census, the third column is age class 16-18 and the fourth column is age class 16-25; therefore, any individual in column three is duplicated in column four.  By subtracting the number in column three from the number in column four, you can create an age class "19-25."

All the children are old enough to have married.  Without knowing their marriage dates and without locating everyone who is outside the home, it's difficult to say just who is still in the household, but certainly a number of the children, their spouses, and children are, not to mention that clergyman are always inclined to taking in the needy, so you typically find "extras" in the household.  Thomas CELLAR's will states that a woman, Betty FISHER, lives with them — are the teenage boys her sons?  The other woman may be George's wife, but it is more likely George is already widowed at this point, which is why he's come back, with his daughter, to live with his mother.  Listed two lines from son-in-law, James GILLIS, in one direction; and, in the other direction, listed next to son, John Flanigan CELLAR, who is listed next to James FLANEGAN, who is listed next to son, Thomas CELLAR, who is listed two lines from Alexander FLANEGIN.

7.  Ruth Bowers & Anita Short.  1989.  "Delaware County, Ohio — Will Abstracts, 1812-1825."  Pp. 383-389 in Vol. I of Gateway to the West.  Genealogical Publ. Co., Baltimore, MD (Broderbund CD-175):

p. 383

The following abstracts were taken from Will Book 1, pages on which record may be found are given in parenthesis (sic)... CELLAR, Thomas -- dated (no date); recorded 4-24-1816.  Wife; Sarah. Sons: Thomas, Robert McCoy, John Flanigan, George, James and Joseph; last three not of age.  Daughters: Margaret, Jane and Hannah.  Mentions: Betty Fisher who makes home with testater.  Executors: Sons, Robert McCoy and Thomas.  Signed: Thomas Cellar.  Witnesses: Joseph Cronkleton, Sr., Joseph Cronkleton, Jr., and Elizaeth Craig Cronkleton.  (4)

The statement that the youngest three sons were "not of age," could not have been true in 1816, so it must be in reference to the date the will was signed.  If so, then the will was drafted and signed before 1812.

8.  Anon.  1880.  History of Delaware County and Ohio.  O.L. Baskin, Chicago (online at Heritage Pursuit):

p. 412	Liberty Township...
p. 416	During the summer [of 1801] a substantial hewed-log house was erected on the site of the present residence of Squire Carpenter.  The family were moved into it and provided with improved furniture and other adjuncts of civilization.  In the spring following Capt. Carpenter's settlement, his party was joined by two other pioneer adventurers, Thomas Cellar and Josiah McKinnie, who were also men of wealth and influence having their land paid for and bringing with them surplus money.  Mr. Cellar had purchased an entire section (4,000 acres) of land (a matter to which we shall again have occasion to refer) and, upon his arrival, built his house near the present residence of E.G. Taggert; McKinnie located on the opposite side of the river from Carpenter.  The colony now consisted of the families of Carpenter, Powers (who came with Carpenter), Cellar and McKinnie.  Cellar was a gunsmith and had manufactured guns for the war of independence while the others had used them to that end.  They were now associated together, not in war, but in subduing the wilderness and building up homes in the new land of promise...
p. 418	Thomas Cellar owned 4,000 acres of land (one section) in the central part of what is now Liberty Township.  He was a native of Franklin County, Penn., and came to his new possession in the spring of 1802.  Josiah McKinnie came with him and hailed from the same region.  The Cases and James Gillies followed a few years later.  These were all related by marriage, or otherwise, and located upon the "Cellar section."  Thomas Cellar had several sons, among them were Thomas, R.M. and J.F. Cellar.  McKinnie was one of the first Associate Judges of Delaware County after its organization.  Both he and the elder Cellar died years ago and sleep in the old burying-ground at Liberty Church.  McKinnie's widow is still living, nearly ninety years old, but quite active.  The Cellar family was a large one, and representatives of it are to be found in many parts of the county...

9.  Anon.  1883.  The History of Hardin County, Ohio.  Warner, Beers, & Co., Chicago (online at Heritage Pursuit):

p. 957

JACKSON TOWNSHIP... REV. T.J. CELLAR. clergyman, Forest, is a grandson of the late Rev. -- Cellar, if not of German birth, certainly of German extraction, and who became a pioneer of Delaware County, Ohio, where he settled in 1802 and remained a resident of until his death.  His son, George, father of the subject of this sketch, was born in Franklin County, Penn., and died in Delaware County, Ohio...  George and his father were both Elders of the Presbyterian Church...

I disagree that the CELLARs are of German descent (please see next Source).

10.  Message #12 at the Cellar Family Genealogy Forum (online at GenForum.com).

11a.  Howard Louis Leckey.  1950.  Tenmile Country and Its Pioneer Families: a Genealogical History of the Upper Monongahela Valley.  7 vols.  Waynesburg Republican, Waynesburg, PA (Sutro Library: F157.M58 L4).  On p. 558:

Family of Thomas SELLARS Sarah Flenniken, daughter of John and Martha (Alexander) Flenniken, was born Sept. 30, 1754.  She died July 18, 1823.  Her husband was Thomas Sellars (Cellar, Keller, etc.) born in Maryland, 1741, probably the son of Johannes Keller, who came to America on the ship Princess Augustus Sept. 16, 1736.  Thomas Sellars removed to Cumberland Co., PA., about 1771, where he served with the Flenniken brothers in Captain John Orbison's Militia Company.  He was twice married, the first wife being Martha McCoy, who died June 19, 1777.  He then married Sarah Flenniken.  After the Revolution, Thomas Sellars and his family removed to Delaware Co., Ohio, about 1800, having sold his land in Franklin County, PA., about that date.  He died in Delaware Co., Ohio, April 16, 1816, aged 75 years.

Phonetically, it's an easy jump from SELLAR to CELLAR, but a very unlikely one from KELLER — the names may look alike, but they don't sound alike.  It appears that the author was too eager to attach an immigrant to this family.  Checking on the arrival of the Princess Augustus on 16 Sep 1736 (see next source), we see that it was a shipload of Palatine Germans arriving in Philadelphia.

11b.  Israel Daniel Rupp.  1876.  A Collection of Upwards of Thirty Thousand Names of German, Swiss, Dutch, French, and Other Immigrants in Pennsylvania from 1727 to 1776...  Philadelphia, PA  (repub. 1931, Leipzig; repub. 1965ff by Genealogical Publ. Co., Baltimore, MD; in 2000 as Broderbund CD-267):

p. 102	42) Sept. 16, 1736. Palatines with their families, in all 330, imported in the ship Princess Augustus, Samuel Merchant, Master, from Rotterdam, Last from Cowes...
p. 103	Johannes Keller sen...

There is no evidence linking Johannes KELLER to our subject.

12.  Virginia Shannon Fendrick.  c1944.  American Revolutionary Soldiers of Franklin County, Pennsylvania.  Historical Works Committee, Franklin County Chapter, Daughters fo the American Revolution, Chambersburg, PA (1982, Microfilming Corp. of America, Sanford, NC).  On p. 189:

Thomas Sellers (Cellars)  Served as Private under Capt. John Orbison, 1780-81.  He mar. Sarah, dau. of John Flanagan, an early pew holder in the Welsh Run Church. Thomas Sellers & wife went to Delaware Co., Ohio.  Penna. Arch. 5th Ser. Vol. 6, p. 264, 274, 294.

13.  Sarah Wilson Cellar.  1890 MS.  A Genealogy of an Ohio Cellar/Cellars Family. 

14.  Marie Sellers Hollinger.  1997.  Sellers Family Genealogy.  [link died]  There were numerous links on this site.  Taking some of them, we find CELLAR(S) equated with German ZELLAR(S), which at least is phonetically plausible (as opposed to KELLER, which is not).  But checking my numerous Maryland data CDs and online data subscriptions, we also find CELLAR(S) of that spelling in Maryland who are Scottish and English.  Please see Message #12 in the Cellar Forum at GenForum.com.

15.  Message posted to the Franklin County, Pennsylvania Message Board (online at Ancestry.com).  States that Thomas was born in Washington Co., MD, which is just across the state line from Franklin Co., PA.

16.  LDS.  Family Search: Internet Genealogy Service:  AF - Ancestral File (online at FamilySearch.org).