They already do at a lot of the stores in my area, and I can tell you it's not as great as it seems. Mostly their prices are terrible! Occasionally they drop the ball and sell games for well under ebay prices (someone local grabbed Mars Matrix for only $50), but 90% of their retro games are between 10-50% higher than ebay.
On a side note, I don't know how they plan on making this work for them. Sure, initially they might be able to produce a decent stock of retro games in their stores since they've been stock piling them for around 5-years now since they began taking games on older platforms, but still, this will mostly be PS2, XBOX, and PS1 games most likely. But the main issue is sustainability. Most of the remaining independent game stores in my area have to turn to ebay lots in order to keep their inventory stocked with games that motivate people to go to their store, with the irony being that those same people could have bought those exact games on ebay for cheaper themselves. At the very best I can only see this idea being profitable for a few months before the supply issue hits them hard and they're back to their predicament of being a failing business.
I hate to say it, but Gamestop's time is running out fast, even faster than I originally predicted. I thought they'd definitely be around for the entirety of next generation, but I am having serious doubts about that. I think it's safe to say that there will be a lot fewer Gamestop's in the coming years as they close off a massive number of stores to stay afloat for as long as possible. For how heavily digital next gen will be, the gen after that will almost certainly be all digital, which means Gamestop is finished as soon as that becomes evident.