Out of those three, I would personally go with FYE myself, since I keep my stuff complete and scratch free most times. Why? Because you can also use their credit online @ fye.com or SamGoody.com too, which is great when you can't find something you want in store.
Only problem with their sites is, they charge $5-6 for the first game for shipping, but if you want dvd's from them, there's a ton of free shipping codes out there. Plus, you CAN switch to budget shipping, which may take up to two weeks to ship to you, but it's cheaper than their normal shipping.
As for Blockbuster, you have to make sure they even TAKE trades, since I know of only ONE BB store around here that does it. Their values are decent, though if you choose to have the credit put on your account @ BB, it's pretty much LOCKED to that store. I've had them transfer it before from store to store, but that's a major league PITA. Plus, their used stuff may tend to look like Gamestops, with stickers, scratches and fingerprints galore on them.
Speaking of Gamestop, as others have said, they give the lowest trade value of the three stores you mentioned, but they are probably the easiest to trade stuff in to. Unless you have current gen games though(360, PS3, Wii), expect to get next to nothing for your games in most cases, as that is their MO.
Their used game selection is pretty expansive, though they charge Ebay-like prices for MANY titles(Baldurs Gate Dark Alliance 2, Marvel Vs. Capcom 2) and the value you'd get for those Ebay priced titles pales in comparison to what you'd get if you Ebay'd them yourself.
Here's a quick recap, in case you don't wanna read through all the crap I just typed up.
FYE= better trade values(25% more for taking CREDIT over cash, no card needed for that), higher prices on MANY used games, but all in all the best of the three imo, you can even use their credit online on their website.
Blockbuster= decent trade values, but not much of a new selection of games and their used ones may be abused previously played store copies, can't use your credit at different stores if you 'lock' it to your BBV acct. You CAN however get it on a 'value card', which is a gift card, which makes it usable at ANY BBV then.
Gamestop= horrid trade values, unless you're trading something hard to find/rare, their used games look like someone took a power sander to them to clean them sometimes and you can't use their credit online either, but their credit is highly portable like FYE's.
Hope this all helps
