The OP is correct that the official definition of the word "monger" means: "A dealer in or trader of a commodity."
However, the use of the word "dealer" throws some gray area into it, imo. Someone who "deals" in any commodity, could be considered either the buyer or the seller, so that would make both parties, client and provider, a monger.
I also don't think it refers to pimps or madams. I promote myself, and handle all my own business dealings, therefore, I am a monger by definition as well.
Another euphemistic term I enjoy for hobbying is "plying my trade".
I personally love language and get pleasure out of being pedantic over semantics, so don't mind someone over-analyzing such things. Proper communication is important!