I skipped through most of the comments, sorry.
Sometimes Gamestop can have really good trade values, it just depends on what you are trading. I've done it, and a bunch of my buddies have done it, where you can trade in loose disc games and get just about what they sell for. I think I traded in Skyward Sword for Wii a year or so back and got at least $40 for it. I know I traded in a loose copy of Sims 3 for Wii for $10 within the past year.
Now back to the main topic. I had a conversation with an SGA a couple of days ago about this, because I thought all stores were accepting trade-ins already. They were previously only accepting certain trade-ins. Now they are accepting ALL trade-ins. Which I think might turn into the getting 25 cents for certain games, but the pokemons and marios will still get moderately good value. I wanna say when it started Super Mario World was paying out higher than it was worth.
Anyway, one of my local stores has been accepting all trade ins for quite some time, and they had approval to sell retro titles in store. I can even take a picture next time I go in if you really want proof. Anything questionable looking, they still send off to corporate for refurbishing and such, but CIB games or cartridge games in good shape, they stay in store. Their inventory hasn't changed in the past few weeks, which could just be all the good stuff getting snatched quickly, but they've had Goldeneye, Starfox 64, a handful of GBA games, a couple of Genesis games, a couple of Xbox games, and Wind Waker for GCN.
Most stores system doesn't have a way of printing out stickers with proper prices for retro games, which is why they can't and won't sell them. But some stores have the proper prices in their system, so when they print out stickers, they CAN sell them in store.
On that same note, I frequently buy retro games from Gamestop, I rarely KEEP the games, given that most folks will trade in loose discs and buying online is total miss or miss, with the ever so infrequent hit. I bought a stack of $20-30 Xbox titles on Gamestop for about $5 a piece, if any came complete, grand, and come loose, oh well it just gets returned. I got my box last night, 1 loose disc, 4 in case no manual. *shrug* Of course, I also ordered a BRAND NEW game off the website, because I didn't want to go find a store that had it and wind up with a gutted copy... so they had a store mail me their gutted copy. Kind of pissed about that.