Selling tracks on-line is a lot of hassle and not worth the effort from the artist's point of view.

Could you explain why?

If the artist is already selling CD's online (as all the artists I've looked-for are) then they already HAVE the merchant account, billing, security, etc, etc set up.

So selling individual tracks should be EASIER than CD's because the tracks have already been mixed, etc, for the CD. All you have to do transcode them to MP3 and bingo.    There's no packaging, pressing, labeling, mailing, etc, to worry about.

Plus you'd VASTLY increase the size of your market because only the richest, most dedicated fans will shell out over $20 to buy one or two tracks.   But they'll happily shell out a buck or two for a track.