Najeeb – There are two problems I see with your idea. Let me say IANAL and I haven’t spoken to any lawyers, so much of my legal knowledge comes from Slashdot. That means you should talk to a lawyer before doing anything because I don’t know shit.
The first problem is that in your situation, the seller wouldn’t be selling the original AAC file or their account, but rather a “derivative work,” which you aren’t allowed to resell. I think. Someone correct me if I’m mistaken, preferably as insultingly as possible 🙂
The second problem is that the AAC encoding process is “lossy” and the MP3 encoding process is “lossy.” This means that you lose some of the information in each encoding process and you wind up with something that sounds worse than either format. It will most likely sound flat and tinny, and who wants to buy that?