This guy offered me a 'Sins of a Solar Empire: Rebellion' key. It was already used. He then offered me three more. All used. He then admitted he'd gotten them in a giveaway. He'd already activated my game and couldn't make restitution because he's flat broke.
We struck this deal: I'd leave him negative rep for essentially scamming me, and he'd refrain from leaving me any. I should also mention that I've traded with him before and it went fine. It seems like he didn't do this on purpose. But be careful trading with him.
I got all these keys on giveaways on the dlh.net website, they were never used, but for some reason I can not figure out they are giving as they have been activated, it was the first time it happened to me in hundreds of trades. Once again I'm sorry for the inconvenience.
This dispute was resolved, I sent him another game of his liking, I leave the following link with the proof in how the dispute was resolved.