Probably my best flip was 2 Kevin Durant basketball cards. When he first came in to the league I bought 2 of his higher end cards, 2 topps chrome white refractor rookies, both numbered to 99. one for I think $22, and the other for about $37. About 2 years later I sold one for $500 on ebay and the other for $450 through PP. Was not a quick flip but definitely paid off.
One good surprise I had was when I bought a lot of about 6 super nintendo games off the bay for around $35, one of the games was supposed to be megaman X, which sells for around $10, but instead I received megaman X3, which sels for around $75. I always like to look for listings listed by people who dont really know what they have. Like when people list an american silver eagle as a "liberty dollar" with a bin of like $1 (of course this will be sniped in all of 3 seconds).