I'm mainly a collector. To my knowledge, I don't think I ever sold or traded in a game. But recently after looking at my vast game collection spanning 20+ years of gaming and seeing the vast majority of these games on the shelves, in boxes or lying around collecting dust, I feel that it is a waste. I have been thinking of starting up a ebay seller account and starting to sell some of my stuff. I own NES games, old-school MS-DOS and Windows games from the 80s and 90s, "modern" era PC games from the '00s, Sega Genesis games, PS1 games, DS games, Xbox 360 games, Wii games, etc.
Though I plan to keep games with high value or rarity. Or potential high value/rarity. For eg. I'm keeping my badly banged up copy of FF7 (I'm not sure if I can fetch much for it anyway though. My jewel case is cracked, my discs are scratched up and have fingerprints. I still have the manual though so that adds some value I guess. I bought FF7 used like 11 years ago (this is why I rarely if ever buy used anymore) and I let an irresponsible friend borrow it (and I don't do that anymore too unless it's a game that doesn't have much future value) so that's why it's in such bad condition. But the discs still run 100% despite being in bad condition.) As well as all the jrpgs I bought this gen on the Xbox 360 just in case they turn out to be valuable. And I'm gonna keep my two import Cave shmups since I'm sure somewhere down the line they are going to be worth a shitload to all the Japanophiles out there.