To grasp the meaning of Chapman, consider the history. The original English spoke German as one would expect Germanic people to do. The key is the phrase 'Anglo-Saxon'. The Anglo component comes from the Angle people who came from Angeln and Engle. Its nearest modern equivalent would be southern Denmark. The language of the Angles was Englisc from which we get English. The Saxon component comes from the Saxons who came from what is now northern Germany. These Germanic people began as mercenaries in what became England in the dying days of Roman Britain and finished as its conquer starting a few decades after the last Roman Legion left England in the 5th century.
The English language was altered by Roman Catholic missionaries who brought their Latin mainly in the 8th century. Around the 9th century, the Vikings took over the north and east of England and many old norse and Danish words changed the English. Then, the Normans altered the language starting in the 11th century. Their status as conquerors is revealed in the English. For example, an Englishman tended to pigs, but the finer cuts went to the Norman masters. So swine and bacon (the opposite of living high on the hog, were least desirable and fit only for an Englishman) were English words. The finished product, mutton is Norman French.
Let us go back to German old English to understand what a Chapman is. Among the German people, the counterpart was the surname Kaufman, which is derived from the old high German word 'chouph'. The old English had several words of similar meaning. 'Cop' meant barter. 'Chipping' was a place where things were bought. 'Ceapian' meant to buy. The old English word 'ceap', also meaning barter, eventually mutated into our modern word cheap. More telling is the old English word 'Ceapman', the old word for a pedlar or merchant, who were usually traveling merchants moving from village to village. It mattered not what specific goods they sold, they were Chapmans.
Chapbooks were thus small books or pamphlets, usually of popular tales, ballads, or poetry, etc., formerly sold on the streets by chapmen.
But be aware, the noun 'chapter' (= a main division of a book, treatise, etc.) has a different origin. It was first used in 1175-1225, coming from Middle English -- var. of chapitre, from Old French, and from the Latin: capitulum (= little head; capit-, s. of caput head + -ulum -ule ). In Late Latin, it meant section of a book; in Medieval Latin, it meant section read at a meeting, hence, the meeting, especially one of canons, hence, a body of canons.