Scammer.

Steamtrades support. (if this had anything to do with steamtrades)
Steam support.

steamrep.com forum

Where's the steamtrades support? Sorry, first time this happened.

If you have supporting documentation of him sending you the fake keys in a trade, just send it to steam support. Hopefully they will ban him for scamming. Because losing your steam account is really the only thing that's an actual punishment. Whatever any other place you report him to do is going to do would just be kinda inconsequential.

I doubt Valve would do anything about it as you were trading for a serial key by the sounds of it which you are recommended to never do.