Never. Ever. EVER sell your stuff to GameStop. I know you don't live near one, but even if you did, don't do it. It is a rip off. Sometimes they won't even give you a full dollar for your games. My friend traded in over forty games at once and he STILL ended up having to pay a few bucks for a brand new copy of Diablo III.
Whew. Now that I've got that off my chest, I'll answer the actual question. Speaking personally, I never sell any of my games (or consoles) because I like to own my stuff. I still have PlayStation games, despite the fact that I haven't touched them in years and a few of them are rather valuable. The thing is, you're probably going to always be able to get access to an old console. Yep. Just checked ebay, and there are people selling original PS and original Xboxes. For pretty cheap too. And these were the first systems, which means you'll be able to get access to a PS3 for a long time.
Games though? Games are another story. You may have a rare game in your collection and you don't even know it. What happens if one day you want to play it again, just out of the blue, and find that it now costs a few hundred bucks? Me, I would kick myself because I can't play it anymore, but there are others who would be upset because they could have gotten more. I never sell my stuff because I like having the option to go back to play it if I want to. It is nice that companies re-release games, but only some games. What happens if they don't re-release the game you want to play?
So no, I say don't sell them, unless you are positive that you don't have that little voice in your head going, "Well...what if...?"