A monger is a seller, especially of something specific like a fish monger or an iron monger.
You can use the noun monger as a word on its own, although it frequently shows up as a suffix, in words like cheesemonger. Monger can also be used as a verb meaning "to sell or peddle." In both cases, the word is a bit old fashioned, used more often these days to describe a person who promotes something hurtful, as in warmonger. The Old English root word is mangere, "merchant or broker," from the Latin mango, "dealer or trader."