I've personally never sold a game I've enjoyed to any capacity and not regretted it. I've never regretted owning a lot of things, but I've almost always regretted selling. Just food for thought.
Then simply don't sell stuff you like to play, even if you haven't played it in a few months.
Unfortunately some of us can't help it. Personally, I lost/sold quite a few greats years ago, and am paying up the nose to get them back. Never again, but I haven't regretted selling, say, Metroid Pinball, or Epic Mickey. I found the games terrible and likely will never go back to them, so it was an easy choice.
It's funny, I just sold Epic Mickey like 2 weeks ago cause I was staring at my collection and all I can think was "Why do I have this game? It blows, I should try to get some money back on it."
As to the other posts, I prolly keep KI Gold and Doom 64 just because they have childhood nostalgia to them. And KI is really my only fighter on the 64 right now, and Doom is still unique even if I have shooters. I do enjoy the collecting aspect of it a lot, but I also don't want to get a game I'm never going to play. I already have these though and like AppleQueso said, I won't get much for them.
Thanks for the suggestions so far guys!
Wiggy: PM me if you want to buy Pokemon Snap. It's complete in box with everything (game, manual, cardboard, even the stickers and other inserts)
Heh. Well it feels good to know I wasen't the only one. I traded it for Kid Icarus Uprising; I think I made a good decision.
Also, I forgot to mention, but "sentimental value" should play a role, just not a huge one. Out of the 400 games I own, maybe 5% of those games are there for "sentimental value", though they're usually excellent games to begin with(Super Mario World, Yoshi's Island, Crash Warped, Sonic Adventure 2, etc).
I just checked Amazon for the value of those two games; Killer Instinct Gold is around $15 to sell and $5 to trade-in, and Doom 64 is $10 and $2 to trade-in. So yeah, if the sentimental value of those games outweight the proposed monetary value, then by all means keep them.
I apologize if I come off as a cunt.