The place or system in which goods are traded illegally it is two words

This would be the Black Market. The Black Market is more of a slang term for a place where goods can be traded illegally. It can be any type of goods and nowadays can take place in real life or online. They tend to happen in specific places where only certain people know about them as of course they are selling illegal goods.

Black Market? this is just a guess but it's two words so yeah