The Walmart thing is interesting, but I'd be willing to bet very short lived. It's already obvious most trades will go to Walmart.com and not the stores. My Walmart stores do not have the room to sell used games, not on the scale of GS. In addition, there are many, many titles that are not popular enough for them to have even sold new, much less used. Pretty much the only games you'll see used in store is Call of Duty, Mario, Assassin's Creed, Resident Evil, etc. Maybe a couple of dozen at best.
The only thing I love about this, is the fact that a trade-in REQUIRES original packaging. If stores like GS or Best Buy did this, people would stop throwing away game cases.