I have a lot of "selling things" regrets.

In 1995 I sold my NES, Genesis, and SNES. . . all with several games to get a Playstation with Tekken.

FuncoLand/Gamestop is pure evil. I wish I just kept everything. They give the absolute worst prices possible.

Letting my sister anywhere near my games is a regret. I gave my copy of Mega Man 2 to a friend because my sister had literally spilled a liter of soda on it. He eventually got it to work. She also traded my NES copy of Snow Brothers for Super Mario Brothers 3 or something extremely common. I'm glad I don't share a state with her anymore.

Oh yeah, I raged out once and kicked a hole in a wall after getting worked in World Heroes. That was stupid.