Really, for light illegal items you can talk to a Black Market Dealer ICly, but you'll rarely ever speak to him in person. You will contact most of the time through a radio or papers, then you can retrieve the items that are purchased in a given location.
Once you make enough sales with the BMD you'll eventually be trusted enough to start purchasing heavier items, maybe a handgun and a box of ammo. Nothing crazy, though. You'll rarely ever get a rifle unless you're in one of the official resistance groups.