I sold/traded most of my 360/PS3 games and even found myself buying the same games more then once. So far with XboxOne/PS4, I've managed to keep most off them and that's what I plan to do. I traded a few to Walmart when they had promos, where I actually received more then I paid for the games. I might trade in order sport games or something like that, but besides that I plan to keep all mine. I currently have 34 PS4 games and 8 Xbox One games. Out of the 42 games I own, about 18 of them are still sealed. I also have multiple digital games for each platform, with GamesWithGold/PS+/EAaccess my digital library continues to grow also.
Really though it just comes down to personal preference. If you want to trade in older games, that way newer games are cheaper for you. Or if you don't mind paying more for newer games, that's good too.
I recently found a website called GameLock, where you purchase a game, and it will tell you a BuyBack amount. You have 60 days to send the game back for that amount. Might be worth looking into for certain games. Some newer games were around $56, with a BuyBack of $40. With that you basically rent the game for $26 for two months. Which is cheaper then say Redbox or Blockbuster(RIP).
What ever you chose, the choice is yours and yours alone. Hopefully something helps you out or gives you an idea of what you want to do.
Sorry for rambling....