There are quite a few Darknet Markets. Within these markets, you can buy practically anything imaginable.
You can buy any imaginable drug, and this goes for both legal and illegal drugs. As on the street, you don't necessarily know what you're getting. Nonetheless, according to some people, regular buyers manage to build relationships with particular dealers, and find the web markets to be safer and more reliable than in person.
You can find any sort of counterfeit items like tobacco, fake money, alcohol, phones, clothing, etc...
You can find all kind of weapons on the darknet, guns, trafficked military goods, automatic rifles, grenades, RPGs, ammo, and body armor among others.
It is possible to purchase stolen identities, fake birth certificates, forged passports, fake driver licenses and much more. This is a scary thing because a great deal of harm could be done if those end up being used by terrorists or such.
You can find lots of illegal pornography, like rape films and snuff films and also child pornography and other child abuse-related content, which is one of the ugliest things you may find there.
You can hire contract killers, but it's difficult to know which are genuine and which are just scams.
You can hire a hacker to do the dirty work for you. They can be found both in the marketplaces and directly on the Tor hacking forum boards.
You can build and host your own .onion service on Tor if you wish as there are many hosting companies. They are not very stable and some of them have been seized in the past so care must be taken.
Besides of this you can also buy legal things on the darknet. There are Bitcoin exchanges, books, software, music downloading sites, etc... This may be convenient to do on the dark web because there is no censorship, the costs are lower and it is highly anonymous.