I was too little to care but we had a boxed copy of Gunstar Heroes for Sega Genesis that my mom sold for two bucks at a garage sale before we moved, what I'd give to have that now… I also remember when I was about three or four me and my younger brother were playing with two NES zappers and our mom told us to go put them back into the box. We put them in a big box which (according to my dad) contained two NES systems with boxes and boxed complete copies of The Legend of Zelda, Wizards and Warriors, Tecmo Super Bowl, Bases Loaded, Mega Man, Mike Tyson's Punch-Out!!!, Super Mario Bros. 3, Tetris, Ice Hockey, Tennis and Golf. There were also two copies of Super Mario Bros./Duck Hunt. I still remember my dad coming home from work and asking what my mom did with the video games, she donated them because my older brothers grades were slipping in school. This was probably 1998 or 1999 so once again I was too little to notice or care. Looking back, I wished I had stopped her though.
My Grandma also gave away her giant SNES collection which had tons of games in it, the likes of which would cost me probably five hundred dollars to buy nowadays. While I do have some of the games from that collection, it still bothers me that I didn't call her when she was moving and maybe ask for a few of them, but that was well before I was into collecting.
Personally, I've given away Luigi's Mansion, Super Smash Bros. Melee and the Legend of Zelda: Ocarina of Time to my older bros. who had them stolen by their POS druggie roommate at the time. Thankfully he didn't take Melee which they gave back to me.
I've lost a few games over time like Sonic Advance, Mario Kart DS, New Super Mario Bros. DS, Donkey Kong Country for GBC (which was actually taken from my desk by my teacher when I wasn't at school and never given back), Super Mario Bros. DX and Dr. Mario for Game Boy. Thankfully most of those games are cheap and already have replacements for. Overall though, not really any regrets on my end. I can remember a time where we went to EB games to try and sell our ten game GameCube collection in order to afford Mario Kart Double Dash but the guy only offered like 42 bucks for all of it which wasn't even enough to buy the game. I remember me and my little bro going home upset that we couldn't get it and we had to wait a while before our dad got us the game. I am so thankful we kept our games, and from that day on, I never let someone try to rip me off again.
Interesting topic, it's good to see that so many of us kept our games and/or never got ripped off.