There are so many places where you can find this marketplaces but you have to make sure youia re using the right now because there are allot of fakers
on the internet who steals your bitcoins.
You got the Tor-network and there is a lot of sold on these so called marketplaces for bitcoins, its the main currency over there without any doubts.
Its just that the bitcoin is untraceable so that is why use it, its easy for criminals and can be done easily anonymous.
The Tor-network is indeed the network that is used for such things but since we cannot do anything to it its not bad that its happening because its maybe bad but it can also boost the bitcoin market if your asking me.