Yet another in the unending stream of ridiculously overbroad U.S. software patents, Amazon now owns the idea of reselling a digital good:
http://www.geekwire.com/2013/amazon-wins-patent-reselling-lending-used-digital-goods/
That would seem to include, for example, Nook-based sharing or any other process that does copy, delete, and reassignment of rights.
Thanks to Library Renewal for sounding the alarm on this one.