"peddler, itinerant tradesman," Middle English form of Old English ceapman "tradesman," from West Germanic compound *kaupman- (source also of Old High German choufman, German Kauffman, Middle Dutch and Dutch koopman), formed with equivalents of man (n.) + West Germanic *kaup- (source also of Old Saxon cop, Old Frisian kap "trade, purchase," Middle Dutch coop, Dutch koop "trade, market, bargain," kauf "trader," Old English ceap "barter, business; a purchase").
This is from Proto-Germanic *kaupōn- (source also of Danish kjøb "purchase, bargain," Old Norse kaup "bargain, pay;" compare also Old Church Slavonic kupiti "to buy," a Germanic loan-word), probably an early Germanic borrowing (Boutkan says 1c. C.E.) from Latin caupo (genitive cauponis) "petty tradesman, huckster, peddler," which is of unknown origin.
Compare cheap (adj.). In Middle English and later, chapman also could mean "a customer, purchaser." In a c. 1200 work the Devil is þe chapmon of helle as "the purchaser of souls."