Why is a dollar called a "buck"?

http://www.etymonline.com/index.php?search=buck&searchmode=term

says:

buck Look up buck at Dictionary.com
O.E. bucca “male goat,” from P.Gmc. *bukkon (cf. M.Du. boc, O.H.G. boc, O.N. bokkr), perhaps from a PIE base *bhugo (cf. Avestan buza “buck, goat,” Arm. buc “lamb”), but some speculate that it is from a lost pre-Gmc. language. Apparently O.E. also had buc “male deer.” The two words (if truly separate) were fully merged by c.1100. Verb is 1848, apparently with a sense of “jump like a buck.” Meaning of “dollar” is 1856, Amer.Eng., perhaps an abbreviation of buckskin, a unit of trade among Indians and Europeans in frontier days, attested in this sense from 1748. Buckshot is first recorded 1447; buck up “cheer up” is from 1844. Pass the buck is first recorded in the lit. sense 1865, Amer.Eng.:

“The ‘buck’ is any inanimate object, usually knife or pencil, which is thrown into a jack pot and temporarily taken by the winner of the pot. Whenever the deal reaches the holder of the ‘buck’, a new jack pot must be made.” [J.W. Keller, “Draw Poker,” 1887]

The fig, sense of “shift responsibility” is first recorded 1912.

er… lets do a better answer:

Though some believe that the term buck for an American dollar originated from the use of silver dollars for bucks ‘markers used in poker,’ others believe that the term buck ‘dollar’ originated due to the fact that buckskins were used in trade, as a form of money, in early America. The term buck was then transferred to currency. If that is the case, then the buck used in poker got its name from the buck ‘dollar’ used as a marker in the game. Unfortunately, no one knows the true answer with certainty

How does ”sawbuck” (slang for a 10 dollar bill) fit in to the overall etymology?

Sawbuck comes from the fact that a sawhorse looks like a big letter “X.” Since “X” is the Roman numeral for 10, a ten-dollar bill was then called a sawbuck.

I heard that it’s because a long time ago you could buy a buck (meat, or skin, or something, i’m not sure…) for a dollar. So people just buck instead of dollar. That sounds kinda strange to me, but it’s what I heard. Not sure if I believe it myself.

I’ve always wondered where using “C-Note” for a 100 dollar bill came from

@jballlou, that’s easy.

oh, duh! Thanks. Both those pieces of information were floating around my brain, but you drew the connection between them.