It is a good idea, imagine buying an album and selling the tracks you don’t like.
I have thought about this before, in fact I just opened up a whitepaper on the idea – tradeusedmusic.com (domain still free).
You have a fundamental right of ownership of a possession, so in theory it should be able to be re-sold, traded, lent, or given away by its owner.
However my idea was predicated on transcoding DRM formats going between different store formats, i.e, Real->WMA or Apple->WMA. This was a problem 3 years ago.
The trend is to go DRM free however, and if you have no DRM then;
a) what value is there in a digital file that can be copied? Is the value of the copy $0?
b) how do you prove you have ownership of the mp3? if a copy is free, can I just sell lots of copies, creating an infinite supply and driving the price near to $0
c) would people think a digital file is ‘used’, there is no wear and tear?
I am sure the majors will stop this, they did it for CD’s before on many occasions – http://www.collegian.psu.edu/archive/1993/08/08-25-93tdc/08-25-93darts-4.asp