If you have any suspicion about it, get a friend to buy a random MP3 from Amazon using her account and buy the same file yourself using your account. Compare the two MP3s using whatever tool you want; I did, and they seemed to be absolutely identical.
FWIW, there is a field in the Amazon MP3 ID3 - under 'Comments' - which says Amazon.com Song ID: 20XXXXXXX (I Xed the last seven digits). That's a unique song identifier, not a user identifier.
I still prefer my MP3s name-free, but that only means that I would pay more for name-free than for named MP3s.
I wonder what the laws are on selling the track and then deleting it though. Is that as legal as selling a cd, or do you get sign some sort of legal agreement when you purchase the track? I've never purchased a single track online so I don't know much about that.
Also, maybe you could just remove the information from the file if it bothers you.
You can probably imagine how silly it would feel to make sure you lose your MP3 after selling it, and what would you even ask for the MP3? Fiddy cent ?