It's just one of the names for the general black market. With no outside regulation, it's easy to get scammed is all.

[A large dog comes trotting out towards the little group, carrying a skewer of some difficult to identify meat in his mouth. Lily grins and crouches, taking the skewer and giving the dog a hearty scratch under his ears.]

Oh! Man, sorry; I haven't introduced myself. I'm Lily; this is Dogmeat.