My Gamecube collection. I sold it because at the time I was jobless and low on cash. If I had known some games would become so valuable in the future I would have kept it. I even had some of the more valuable games like Skies of Arcadia, MGS: Twin Snakes, Fire Emblem, Eternal Darkness, the Pokemon games, etc.
Also had a Virtual Boy I had gotten at a garage sale for $1. I'm more upset with that one because my mom donated it. That also skyrocketed in price