yogsloth: I just want to add I've made multiple trades with this user, and have had no problems. I just manually re-checked, and all seven games I've traded for are still on my shelf. I would contact the user directly and find out what's going on.
I'm in a conversation with the user right now, will post updates when I have any.
Update #1:
The user says the keys were indeed from an untrustworthy source, but he had no reason to believe they were going to be revoked because whenever he got gifts from there, they were working fine. I don't believe they had any intention to mislead or defraud, and were just a little careless. We're currently in the process of working out some kind of solution.
Update #2:
The thing has been resolved. They offered to recompense me and we found a good compromise. I really don't believe they ever expected this to happen.