I hate Gamestop as much as anyone, but I don't see why people are so worried about this. Gamestop is totally doomed to fail at this...but more importantly, if they DO succeed, what's the worst that can happen? eBay loses its virtual monopoly on retro stuff and the prices go down? I see that as a good thing.
Not that its going to happen, of course. Even successful indie game shops have to resort to eBay quite often.
What could go horribly wrong though, I suppose, is if GS just uses it 1000 stores to collect as many retro games as possible and then sell them exclusively on eBay. I don't see this happening.