I'm in the same boat, actually. My kids have a GBA, and since getting my DSi I have a newfound appreciation for the device, in large part because it also plays GB and GBC games, which is something even the DS Lite can't do.
I've been paying Super Mario Land 2 on their GBA. Awesome game. But now I need my own GBA. Definitely want a backlit one. Plus maybe then we can get a copy of Four Swords for NGC finally. Is it possible to check a GBA at gamestop before buying, even look through a bunch of them, or are you just buying a box and hoping what's inside is good? I've never bought a used console at GS before. Also, do you get 10% off with Edge, or does that not apply? Lastly, is there any way to tell from just looking at a GBA that it is backlit? For example, if buying on ebay.