Green’s Dictionary of Slang

a dishonest horse-trader who buys stolen horses and disposes of them at fairs.

1591		Greene Second Part of Conny-Catching in Grosart (1881–3) X 76: The Martar is he that receiues him [i.e. a horse], and chops and chaungeth him away in any Faire, Mart, or tother place where any good vent for horses is.
1608		Dekker Belman of London G1: The seller away of the stolne [sic] horse is a Martar.