Old English céapmann was the regular term for "dealer, seller", cognate with the Dutch koopman and German Kaufmann with the same meaning.
The word also appears in names such as Cheapside, Eastcheap, Chepstow and the prefix Chipping: all markets or dealing places.
The name of the Danish capital Copenhagen has a similar origin, being derived from Køpmannæhafn, meaning "merchants' harbour" or "buyer's haven".
By 1600, the word chapman had come to be applied to an itinerant dealer in particular, but it remained in use for "customer, buyer" as well as "merchant" in the 17th and 18th centuries.
[1] Their stock in trade provides a graphic insight into the methods of political and religious campaigners of the Civil War period, for example.