This document (Adobe Acrobat format) outlines a legal ruing in the EU that seems to open the door for resale of digitally distributed software (thanks Joao). Here's a bit:

Where the copyright holder makes available to his customer a copy – tangible or intangible – and at the same time concludes, in return form payment of a fee, a licence agreement granting the customer the right to use that copy for an unlimited period, that rightholder sells the copy to the customer and thus exhausts his exclusive distribution right. Such a transaction involves a transfer of the right of ownership of the copy. Therefore, even if the licence agreement prohibits a further transfer, the rightholder can no longer oppose the resale of that copy.