I don't know.

Drugs, guns and any thing you don't want your gov to know about.
Maybe poison?

Cool hacking gear. Like a remote to turn red lights green. super charged lazers. GPS and wireless blockers. (for stealing cars) and other stuff
all you need to do is ask what do I want to buy or what i don't want a record of me buying or that I am told I can't buy.

Black-powder, Kits to make moonshine, super endangered wild life to be sold as pets... the list go's on and on.