Selling my CIB Genesis and around 5 CIB games (including Fire Shark and Splatterhouse 2) to some crappy game store to purchase Star Fox for the SNES! Yeah, that's right.... all of that to purchase ONE SNES game! And this was in '93!
Selling my PSX and all 20-something CIB games (including Lunar: SSSC, Einhander, Time Crisis, etc) for, maybe $150 or so to pay for part of the rent, only to land a decent job 3 days later, which would've negated this whole need to sell my games in the first place. I actually have re-acquired all the prized gems that I sold (except that CIB Time Crisis set), but I still think that this was probably the dumbest move that I have ever made when it comes to games.