Proposed: The Internet Library Circulation Department
I have a copy of a book. I would rather have it in PDF format so I can search it for specific terms and store it electronically. So I rip it apart and I scan it. Later, I’m done with it. I sell or donate it to the Internet Library Circulation Department (ILCD) by sending it by e-mail or on CD.
At this point, my rights in the book cease, just as if I had sold or donated a physical book. The ILCD could be set up to verify that my copy of the PDF had indeed been equipped with a neutering tag, rendering it unopenable.
The copy at the ILCD would be available to other borrowers, just like a real book. While it was in use by one borrower, it would not be available to others. The electronic versino becomes the complete replacement for the physical version.