You realize anything they have sitting around in a warehouse is likely to just be some scratched-up/refurbished disc only PS1 sports titles and ex-rental cart-only shovelware N64 games with mauled labels, right?
Go to your local Gamestop and look at what they have for PSP, PS2, and Gamecube, if your store is fortunate enough to even still carry those. You'll find a bunch of awful, unwanted games in terrible condition. Anything worthwhile will have been picked through and bought months if not years ago at this point. 99% of anything worthwhile they had for old systems would have been sold when they firesaled this stuff originally.
As for trading in old stuff, I remember how they once tried to sell me a PS2 copy of Silent Hill 2 where some f*cktard had carved his name into the data side of the disc. Or the time as a kid they sold me two obviously broken NES games that were missing large chunks of their casings.
Gamestop has never card about the condition of the item you trade in (and neither did FuncoLand for the majority of its existence in these parts). Hell, the local stores around here still throw away cases and manuals on older games under a certain price in the name of saving space, apparently unaware that doing so will make it even harder for them to sell those games (because people like me wont buy an incomplete game). They'll give you the same $2 store credit for your Earthbound regardless if its CIB or a mangled, yellowing, stained, sticker-cover, missing-label POS with parts of the casing missing.