Serious question, not trying to 'flame' the issue!
And a good point - in some respects I can see no difference between a book and a CD; both are physical artefacts recording another's creative input. Much of my music when I was younger was garnered from the trade-in bins of record shops (there used to be a superb place in Notting Hill that had the most wonderful and obscure LPs).
The significant difference, however, is that over the past decade technology has made the copying of music (and video) - far simpler and thus the notion of 'intellectual property' more important. Ron Olesko seems to have got to the crux of the issue - if you keep the content and trade the artefact, that is morally wrong, but if you simply trade in a CD or LP you no longer want without copying it, 'title' passes to the new owner, and the original copyright owner has no rights in the matter.
Compare it to a painting - if an artist sells a print, s/he makes money on the sale. But if the new owner sells it on, then the artist has no right to come back and demand a cut from that transaction. If, however, that new owner makes copies of the print and distributes them, the artist has a right to prevent this.