(By 1600, the occupational name chapman had come to be applied to an itinerant dealer in particular, but it remained in use for both "customer, buyer" and "merchant" in the 17th and 18th centuries.
Modern chiefly British slang chap “man" arose from the use of the abbreviated word to mean a customer, one with whom to bargain.)
The OED includes a citation of an English ordinance or decree that dates from 1553, during the reign of Edward VI: "No Tinker, Peddler, or petit Chapman shall wander about from the Towne but such as shall be licensed by two Justices of Peace."
Both the compound “chapman” and its first element chap- have cognates in all the major Germanic languages: From the prehistoric West Germanic compound *kaup- are derived cognates Old Saxon cop, Old Frisian kap "trade, purchase," Middle Dutch coop, modern Dutch kopen “ to buy,” koop "trade, market, bargain and goedkoop “inexpensive."
The common ancestor is Proto-Germanic *kaupoz-, which was probably an ancient Germanic borrowing of Latin caupo, caupon- "petty tradesman, huckster," of unknown ulterior etymology.