I saw a lots of players who are selling another player set but he didnt bought it, we can sell sets if wwe dont bought it fix me with this question

You can sell art, what you have bought or made by yourself, but you can not sell someone other's set without access. If you do, it can lead up to banning you. called plagarism.

when you see some one doing that talk to the owner and if he says he didnt sold it just use the report button ;)