Sometimes you don't realize how much you like something until it's gone. Maybe you have a friend that trades you their game for yours or perhaps you just sold off a game at a store for a few $$. Either case, later on you may have regretted getting rid of them.

So what game(s) was/were that for you? 

For me it's definitely be Mystical Ninja Starring Goemon for the N64. It's not the best game ever but it has a certain charm to it that I love. One day though a friend of mine offered to take it from me in exchange for 3 crappy games. I was 8 at the time and didn't realize I was screwing myself over in the deal. On top of that one of the games was for a system I didn't even own!.. Oh god, young me was an idiot. Well I never traded it back and I eventually moved away. Nearly 10 years later I found a copy of the game for $45 at a retro game store and I love it v.v XD 

A runner up is Super Smash Bros. Melee and Pokemon Stadium 2. I'm not sure why I traded/sold these games -I did eventually get Melee back but I've yet to find a fairly priced Stadium 2

FFVII Crisis Core. Loved that game to death but sold it with my psp in order to get money for my vita. Kinda missing my psp now along with crisis core. Oh well, I guess thats what emulators are for :P

This is a long list.

Most of my Gamecube library including Luigi's Mansion, Melee, Pikmin, Pikmin 2, Super Mario Sunshine, Mario Party 6. Ended up rebuying most games at much higher cost.

For Wii, I regret selling Super Paper Mario and Monster Hunter Tri. The latter was a rage sell.

Selling my Super Mario RPG cartridge was one of my biggest mistakes, ever. ;-;

starfox adventures
star wars rogue squadrom 2
wave race blue storm

i still hate myself ..

My entire NES collection. Gave it away when I was changing home. It was a mistake.

: ( most of my N64 games... I miss them, like Banjo Kazooie and the Mario Party games...

Mario sunshine...young zippy was such an idiot :(

BF4, it really was great and there was more stuff to do than hardline. And now they're releasing free content.

I don't really recall me doing much of that, probably when my parents sold all of my Wii-related things T_T

I do remember when I bought a Gameboy Micro for £10 from a friend, glad I did that :P