A lot of those sellers are kinda sketchy if you ask me. I have a friend who bought a Windows Server 2000 license and the COA wasn't what you'd call a normal COA... Though he insisted it was. I was very skeptical, and tried to argue with the guy on behalf of my coworker. I could never get a straight answer as to why the COA looked different than for say a normal XP sticker COA...
So anyway, I personally wouldn't trust those sellers. If you absolutely positively need legit licensed software, then buy retail and save the headache.