I've had the reverse happen. I thought I sold a game but I didn't. Two of them, actually. I seem to remember selling my copy of Gungrave: Overdose about a year ago, but I was looking through the trunk of my car recently and it was in there. I thought I also remembered selling my copy of Legaia 2: Duel Saga but it was in a plastic container in my room with a bunch of other games.
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I don't trust anyone because I'm cynical.
I'm cynical because I don't trust anyone.