I am unfortunately more often the guy who buys high and sells low... but I have stolen a few in my time as well...
1. Willie Mays Original Hartland Statue (Mint) for $2 at a Thrift Shop (I begged them to check the back room to see if there were any more... no such luck!)
2. Rare Tony Perez 1968 Topps Experimental 3D Card for $15 in Ex/MT Condition
3. Brooklyn Dodgers 1950s Clown/Bum Pennant (NRMT) for $3 at a local Card Shop
4. 1954 NY Giants Champions Scroll Pennant (NRMT) for $3 (at the same store!)
The funny part is that both of those pennants are worth over $200... he also had a very common Pirates Pennant, which normally goes for about $30. He wanted $50 for that one, because he thought it "looked so cool". He was surprised I passed on it, but was happy to get the $6 total, for the Bums and Giants