Mostly illegal stuffs; illegal porn, illegal drugs, illegal fire arms, warez and carded & carding related stuffs. I've been there on some black markets out of curiosity and all you can see were illegal shits but of course I never attempted to purchase any of them cuz of only two possibilities; you'll either be scammed or be jailed They do all accept bitcoin as a payment cuz of its "anonymous" nature.

P.S. It's not advisable to go in any dark web sites without any safety measures. Browse there at your own risk