Green’s Dictionary of Slang

buffer n.1

[SE buff, the skin; see 1818 W. Parry London Guide 92: The term buffer derived from the pratice which once prevailed of carrying Bandanas, Sarsnets, French stockings, &c., next their shirts; so, as they were obliged to undress in order to come at the goods, or in other words, to strip to the skin or buff they obtained the name of buffers]
(UK Und.)

1. [late 17C–mid-19C] a villain who kills healthy horses and sells the skins; also a dog-thief.

2. [mid-18C–mid-19C] (also buffor) a villain, specializing in selling supposedly smuggled goods; thus buff it, to peddle such articles.