If you're gonna buy a used drive, your best bet is GameStop, even if you don't have an EDGE card. That way, you can bring it home right on the spot and test it out for a day or two. If it's a dud, you can quickly and easily return it. If you get a dud from eBay or Amazon, you gotta wait awhile for it to come, and then you got gotta fart around returning it. To me, it's worth it to spend a few more bucks and have that that convenience. Or, you could just get an EDGE subscription, save the $10 on the drive, and then you'd basically be getting a year of EDGE for $5, which I'm sure you can find a way to use.
As far as buying drives online goes, watch out where you are buying it from. Many of these are drives are mods. Someone buys a bunch of broken drives, so that they can get their hands on the casing. Then, they buy the model of Western Digital drive that MS uses. They should work, but frankly, I'd pass on stuff like that.