So many I have lost count.... many a one where I either bought a game at full price, or passed on a game (or even game merchandise, toys, books, etc) and came to miss out entirely because of it... Among my most regretted full price purchases, certainly Where's Waldo and Ghostbusters 2 for the NES, and Mega Man's Soccer for the SNES (not because I wouldn't have bought it anyway, but because I bought it for $60 and a month later I saw 6 copies in the discount bin for $15 each. Ack...) I still continue this trend today, since I got MvC3 late but at the same price as when it was new, and less than a month before the Ultimate version was announced.
I've never known the heartbreak of making a bad trade, because I have never traded or sold any of my games or systems, ever. Even the ones I hated or only got because they were bought as a lot on Ebay. That said, I've lost and broken a few...