Sorry to bump the thread but I just found this while googling to make sure I wasn't setting myself up for a problem by selling my iTunes card on Ebay, and thought I would offer an alternative opinion as to why you might find cards listed for prices less than their value (i.e. ostensibly losing money).
Some people use... other methods of adding to their digital music library.  Sometimes these people have well-meaning friends and family who either do not realize this, or who are lawyers and therefore do not approve of such activities.  Either way, a person may get a gift card to iTunes that is completely useless to them, particularly if they are a student who needs cash.
I, for instance, am about to list a $50 gift card for sale, so I wanted to thank those in the thread above who mentioned that they have had a successful transaction.