Meh . . . I broke my "no ordering used games from the GameStop website sight-unseen" rule a couple of weeks ago because of the guarantee and ordered a used game. Sure, it did indeed come with a cover and a manual. The disc was scratched, the manual was beat to all hell, and the cover had some unidentifiable black crap stuck on it. Needless to say, that shit went straight back to the nearest store, and my rule went right back into effect, guarantee or no.
Unless they're going to guarantee the condition of everything as well, their guarantee is worthless to me anyway. They might as well kill it.