Couldn't you explain that you got the key from Steam Gifts and that you were not aware it was a leaked key?

There was a giveaway from some company iirc, and a lot of them were given out when buying a Raedon gpu.

http://www.steamcalculator.com/id/ThatGuyHats%25252F

Dunno if OP would risk his account for this type of thing.

/Edit2 Talking with the winner, he said his key is on the pastebin list below.

I suggest opening a ticket with Valve asking if the key is legit, if it's not, link them to Op's profile and this giveaway.