Well, here is my story. I was looking for spotify codes in the marketplace and saw that "d3m0n1q_733rz" guy pming me about having one he would sell me for 20$. Because this sounded quite suspicious I asked for an escrow. Since then I've never got an answer. This doesn't proof anything but it's still suspicious.
Further private message exchange shows that this user seems non-legit as the seller presumably cannot provide a proof that he actually is in the belonging of the spotify codes he was going to sell me.