Is it Legal to Resell Your Digital Music?

ReDigi has created software that purports to remove old digital music files from the user's equipment and then allow that user to resell these files through ReDigi's on-line marketplace. Capitol Records filed suit against the company in 2012 claiming copyright infringement. The first sale doctrine seems to suggest that a purchased copy can be legally sold. But is a digital copy a true "copy" under copyright law?

UPDATE: In April 2013 the Court granted summary judgment for Capitol Records, finding that ReDigi's online sale of digital music constitutes copyright infringement. This ruling creates a distinction between physical items and digital ones. While this case applies to digital music, it may affect the resale of all digital works including e-books. ReDigi may appeal.