Old English mangere "merchant, trader, broker," agent noun from mangian "to traffic, trade," from Proto-Germanic *mangojan (source also of Old Saxon mangon, Old Norse mangari "monger, higgler"), from Latin mango (genitive mangonis) "dealer, trader, slave-dealer," which is related to mangonium "displaying of wares."
Not in Watkins or de Vaan, but Buck (with Tucker) describes it as "one who adorns his wares to give them an appearance of greater value" and writes it is probably a loan-word based on Greek manganon "means of charming or bewitching." Used in combinations in English at least since 12c. (fishmonger, cheesemonger, etc.); since 16c. chiefly with overtones of petty and disreputable (for example ballad-monger "inferior poet," 1590s; scandal-monger).
"to traffic in, deal in," often implying a petty or disagreeable traffic, by 1897, from monger (n.). Not considered to be from Old English mangian. Related: Mongered; mongering (1846).
1510s, "itinerant apple-seller" from coster (see costard) + monger (n.). Sense extended from "apple-seller" to "hawker of fruits and vegetables," to any salesman who plied his wares from a street-cart. Contemptuous use is as old as Shakespeare ("Virtue is of so little regard in these coster-monger times, that true valour is turn'd bear-herd" "2 Henry IV"), but the reason for it is unclear.