Actually, their trade in FAQ which they print off the Wire, which is their intranet, specifically says they aren't supposed to put the egift credit on a WalMart gift card. I assume this is probably to track where that specific type of credit is being spent. So that is very YMMV. But, if you spent $100 trade in credit and bought a pack of gum whats left would go onto a WalMart gift card. Also, WalMart policy on any kind of credit is that it is NOT supposed to be used with prepaid visa cards, so again that is YMMV.
Also, there is a $250 trade in credit limit per transaction, and it also specifically states that they are not supposed to take in games without UPC codes. They can search without it though in the system, as for the most part it looms exactly like what we see on the trade in web page, so pack in games like two copies of the gears of war 3 I traded in last night might not be accepted, so YMMV. they would not take mine.