I will always be on the lookout for rare stuff in thrift stores and garage sales, but there is very little I am willing to pay "market value" for.
I am currently trying to collect all PS1 RPGs and the only really big ones I have left are the Lunar games, Arc the Lad Collection, and Vanguard bandits. The other 15 or so I am missing are sub $50, so not really "rare".
The only other rare titles I can think of that I really wish I had (but still won't pay for) are Snow Bros., Mega Man 7, Mega Man X3, Shining Force 2 and the Misadventures of Tron Bonne