Margaret is said by some to be a sister of Lucy HUFFMAN, but Margaret is not mentioned in Henry HUFFMAN's 1827 will — where Lucy MARKSBURY is mentioned.  More importantly, Lucy's mother, Milly, would have been much too old to have birthed a child in ca. 1816, because she would have been some 56 years old at the time.  There is a strong case to be made — and some secondary sources show — that she's the daughter of Joshua KELLEY.  Note that Joshua is living with her in the 1860 census, that he is known to have had a daughter Margaret's age, and that Margaret named a son, Joshua.  If Joshua's wife was, indeed, Rebekah BROWN, then Margaret named a daughter and son, in succession, for her parents.

— born in KY or IN:
  2.  (Son A) MARKSBURY, b. 1820-25
  3.  (Son B) MARKSBURY, b. 1825-30
  4.  (Son C) MARKSBURY, b. 1825-30
  5.  (Daughter A) MARKSBURY, b. 1825-30
  6.  (Daughter B) MARKSBURY, b. 1830-35

— born in IN:
  7.  Samuel MARKSBURY, b. 1832/3
  8.  Lucinda MARKSBURY, b. 11 Dec 1836, Putnam Co., IN
  9.  (Son D) MARKSBURY, b. 1835-40
10.  Eliza J. MARKSBURY, b. 1838/9

Unless we find a record of Lucy's death or of Margaret's marriage, we can't really be certain which children belong to which mother — although, based on the age of the wife in the 1840 census, she would appear to be Lucy, so that only brings into question the mother of the children born between 1840 and 1850.  Margaret's apparent father was named, Joshua, so I think we can be certain Joshua and his younger siblings are Margaret's.  That leaves only the three above uncertain.

1.  Jordan R. Dodd, ed.  1993.  Kentucky Marriages:  Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Marksberry, Isaac	Radcliff, Susannah	17 Jan 1794	Lincoln Co.
Marksberry, Richard	Huffman, Lucy	20 Jan 1819	Lincoln Co.

2.  1820 Census Index/Images (online at Ancestry.com, Image #27 of 36; extracted by Diana Gale Matthiesen):

1820

KY

Garrard Co.

Garrard

Roll M33_21

p. 114-115

Ln. 8

Richd Marksbury

000100-10100-0-100

no slaves

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	19-25*	1794-1801	= Richard (b. 1801)
1 female	9 or under	1810-1820	= Elizabeth (b. 1820)
1 female	16-25	1794-1804	= Lucy (b. ca. 1801)
1	person engaged in agriculture
*In the 1820 Census, the third column is age class 16-18 and the fourth column is age class 16-25, which means any individual in column three is duplicated in column four.  So, for an accurate representation of the household, the number in column three should be subtracted from the number in column four. If, after the subtraction, any individuals remain in column four, then they have to be age 19-25, thus creating a "new," more precise age class.

Listed two lines below Huston HUFMAN; Henry HUFMAN is on the previous page.

3.  1830 Census Index/Images (online at Ancestry.com):  can't find.

4.  1840 Census Index/Images (online at Ancestry.com, Image #3-4 of 20; extracted by Diana Gale Matthiesen):

1840

IN

Putnam Co.

Washington Twp.

Roll 91

pp. 340B-341A

Ln. 18

Richard Marksbury

112 101 - 211 001

0300000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	4 or under	1835-1840	= Son D
1 male	5-9	1830-1835	= Samuel (b. 1832/3)
2 males	10-14	1825-1830	= Son C = Son B
1 male	15-19	1820-1825	= Son A
1 male	30-39	1800-1810	= Richard (b. 1801)
2 females	4 or under	1835-1840	= Eliza (b. 1838/9) = Lucinda (b. 1835/6)
1 female	5-9	1830-1835	= Daughter B
1 female	10-14	1825-1830	= Daughter A
1 female	30-39	1800-1810	= Lucy (b. ca. 1801)
3	persons employed in agriculture

5.  1850 Census Every-Name-Index/Images (online at Ancestry.com, Image #35 of 42):  Washington Twp., Owen Co., IN, Roll M432_164, p. 101A, 243/243, enumerated 26 Aug 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):

Richard Marksbury	47	M		Farmer	2000	Kentucky
Margaretta	33	F				Do
Samuel	17	M		Farmer		Indiana
Lucinda	14	F				Do
Eliza J	11	F				Do
George	8	M				Do
Richard	6	M				Do
Rebecca	3	F				Do
Joshua	2	M				Do

6a.  1860 Census Every-Name-Index/Images (online at Ancestry.com, Image #2 of 22):  Gosport P.O., Wayne Twp., Owen Co., IN, Roll M653_287, p. 156, PN 156, 1107/1098, enumerated 26 Jul 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):

Margret Maxbery	42	F			200	50	Ind
Rachel    "	9	F					"
Joshua    "	11	M					"
Joshua Kelly	70	M					Ky
Matilda  "	22	F					Ind
Jane Green	20	F					"
A J Hays	16	M		Book Keeper	2500		"

Joshua KELLEY is said to be Margaret's father, and his living with her supports that claim, as does her having named a son, Joshua.

6b.  1860 Census Every-Name-Index/Images (online at Ancestry.com, Image #43 of 45):  Gosport P.O., The East Division, Owen Co., IN, Roll M653_287, p. 50, PN 50, 364/354, enumerated 13 Jun 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):

Benj Hancock	30	M		Farmer		200	Tenn
Margret  "	28	F					Ind
Mary J   "	11	F					"
Geo Maxbery	16	M					"
Richd   "	14	M					"
Mary    "	8	F					"

If just George and Richard were living out, I'd say that was a clear sign they were Lucy's children.  However, that doesn't explain Mary, who would be the youngest and surely Margaret's child.  I have yet to find a genealogical link between our subjects and Benjamin HANCOCK or his wife.

7a.  1870 Census Every-Name-Index/Images (online at Ancestry.com, Image #23 of 35):  Gosport P.O., Wayne Twp., Owen Co., IN, Roll M593_348, p. 383A, PN 23, 184/189, enumerated 8 Jul 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):

Rader Geo W	32	M	W	Comercial ha??		500	Ohio
_____ Rachel	18	F	W	Keeping house			Ind
Marksbury Margaret	46	F	W	House Keeper			Ind

7b.  1870 Census Every-Name-Index/Images (online at Ancestry.com):  George MARKSBURY (æ 28, b. IN) is unmarried and living in Wayne Twp., Owen Co., IN; Josh MARKSBURY (æ 21, b. IN) is married and also living in Wayne Twp., Owen Co., IN.

8.  1880 Census Every-Name-Index/Images (online at Ancestry.com):  Margaret not found.  Joshua MARKESBERRY (æ 29, b. IN) is still married and still living in Gosport, Wayne Twp., Owen Co., IN; his is the only MARKSBURY family left in the county.

9.  LDS.  Family Search: Internet Genealogy Service: Pedigree Resource File (index to CDs, online at FamilySearch.org).

10.  LDS.  Family Search: Internet Genealogy Service:  IGI - International Genealogical Index (online at FamilySearch.org).

11.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).