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).