I used to keep them all, but then I realized "...why?" half of these of beaten, some of these I will never bother finishing, and in some very rare cases I'll find a game that I never even played once...
I started selling them to a used media store called 2nd&Charles that opened nearby for store credit (they give a lot fairer prices than a place like GameStop, and I can use the credit to buy a lot more things like games, movies, books, and anything else in the store)
Hell, with all the books and games I traded in, I got myself a 3DS, and lots of other stuff.