A fourth, unproven claim is that the term originates from the bloody scalps (red-skins) of Native people taken for bounty prizes after battle, and their skins bought and sold in local towns. There is no historical documentation or evidence to support this theory, which is generally taken as an urban legend.
From "The Idea of the Indian: Invention and Perpetuation," from "The White Man's Indian: Images of the American Indian from Columbus to the Present" by Robert F. Berkhofer Jr. (First Vintage Books Edition, 1979; originally published by Alfred A. Knoft Inc., 1978).
REDSKIN--"redskin, 1699; red man, 1725; red devil, 1834." From "I Hear America Talking" by Stuart Berg Flexner (Von Nostrand Reinhold Co., New York, 1976).
For more on Redskins as a sports team name, see Campaign Against Nepean Redskins and Kansas City Star Won't Use Redskins.