Even though we didn't trade anything in the end, I must give this user +REP. I tried trading with him, his game - Sora for my 4000 gems. I sent them the gems, got the key, I tried to activate the key.. but it said that the key is duplicate. So I told the seller and he was like "Oh, this never happened to me before.. sorry, I'll give you your gems back." so he did. He contacted steam support for it, but sadly they didn't figure anything out. All I can do is to wish this seller a great future trades and hope that this was one time life experience.
Kind reliable trustworthy and patient trader^^
+rep, quick and easy ! traded speedrunners, all went fine. Thanks for the trade :D