"let customers sell their previously read eBooks, audiobooks, music and movies the same way that consumers can now sell print books, DVDs and CDs, ..." (source)
(where I originally read about this)
Redigi is a site that's already trying for legal resale of digital music, with the assurance that the song is properly removed from the original owner's devices when sold. It's not much of a stretch to extend this to ebooks.
I highly recommend Nathan's article, which goes over some of the pros/cons of such a service as well as links to some very interesting responses.