Here's how the Fort Vancouver Regional Library (WA) deals with weeded (excuse me, deselected) books: They box them up randomly and offer them to bidders who have no idea what they'll be getting. The library made a little over $2100 on 76 boxes of books...at least enough to recoup the cost of advertising. There's also some explanation of why and how libraries weed (excuse me, deselect). More here from The Columbian.
Here's another lengthy, older piece on weeding that I found this weekend, as the library where I work is about to undergo a serious weeding (excuse me, deselecting) project. It's from Bad Subjects: Political Education for Everyday Life.