People who download MP3s from unknown sources are risking poor sound quality due to the way the MP3 might of been created.
If MP3s are for sale surely they must be encoded in the best way possible, ie 320kbit with lame encoder (how can you trust everyone to do that?)
No way can you sell a track as a 128kbit MP3, ive heard of people already doing this...
In my world people would sell their music online compressed as lossless FLAC files, once you have downloaded / paid for the music it would be then your choice to burn a CD or create a MP3 or whatever...