I keep all of them for now, I love collecting games and space/money isn't really an issue for now.

I'm too sentimental to ever trade in or return most games.

I'll keep all my console games most of the time.
But, every once in a while a store will have some sort of trade-in promotion that I'll take advantage of.

I keep only the ones that I like.

Never sold any of my games, be it on consoles or PC. Still got loads of 5.25 Floppies and 3.5 disquettes here, even though the computer to play them is out of order atm (needs a "new" ISA Graphics card). My NES also doesn't seem to work anymore, but that doesn't mean I'll sell those games.

I still play lots of older games on older consoles, mostly SNES and Sega Master System II. On PC, I also have 6 times more games on my shelves at GOG.com than at Steam because most modern games just bore me or are simpy not of the genres I like to play

I wondered why the community warned me about loaning you games when I joined ...  ;)

I keep them in fact, I'm running out of space to store my collector's editions

The only games i gave away were amstrad cpc because it was broken & NES & Master System because i gave them to cousins of mine.
From Playstation 1 Era and after I collect them.

I keep most of mine, but I'll sell ones I didn't like, or have no intention to finish or replay.

keep for sure in general. I usually only buy games that I am fairly certain I'll dig though, and when it comes to classic Nintendo there are a lot of those (or even PS1, Genesis, etc.)