Valve has come out and refuted the notion
of a trade or exchange service coming to Steam. A quick it's untrue from Doug Lombardie of Valve has squashed any notion of Steam deploying said feature.
I'd still love to see something like this though, even if there are no plans. I just had a discussion with a few folks today about how they stopped spending money for games because they cost so much and wished they could get something back for their digital purchases.
So, while Valve says nothing of this is coming, I hope they are thinking about it.