Feel free to add to this thread in the replies. the black market has got kind of dull. The most you can do is weapons or drugs.
On an old server I used to play on (it shut down because not enough devs) there was an illegal organ trade. At the clinic, you can buy scalpels, tubs, ice packs, etc., and when someone is restrained, you can harvest their kidneys and sell them at the clinic/black market for a high price.
As a deterrent to prevent people from overusing this feature, you could add a large bounty/news broadcast for whoever takes an organ from someone else.