I think that it would make the most sense if the black market was comprised of (concerning military surplus at least) military brass, and other people who would have also had the means to hold on to a large amount of military surplus. Though i would also say that it is most fitting for organized crime syndicates to be the ones dealing in slaves drugs ect ect. I just cant see crime syndicates having the means to obtain military surplus, given that at the time of the worlds decline, present day currencies would decline as well. Currency would switch to something which has actual value, and so crime syndicates would only have drugs, prostitutes, and a relatively small amount of weapons to use as bargaining chips in the new world. I do realise that drugs are nothing to scoff at as a commodity, but if you've ever seen a crime movie, they're usually more trouble than they're worth when it comes to a means of payment.
people will always have vices, and people will always want to wage wars and kill each other . Due to this and the world that the game takes place in, the black market would really be divided into a few mutually exclusive, abstract entities: crime syndicates who deal in poor quality weapons and serving vices,the "rouge" members of the former military brass who were in convenient positions at the time that big government would have dissolved, and shadowy characters who deal in the acquisition and sale of relics and other artifacts. Unfortunately the idea of the "black market" being made up of biker aesthetic thugs isn't really a rational conclusion, and doesn't make the idea of the black market into a very complicated story piece.