The head of my local Friends of the [name of town] Public Library is extremely picky about what books she allows in the library book sales.

Science books, especially medical books, can't be more than a year or two old (unless they are popular reads like The Hot Zone or The Immortal Life of Henrietta Lacks.) Same with diet and nutrition books.

Other sciences are usually reliant on using up-to-date technology, so it doesn't serve students very well to show textbooks featuring methods used on machines from the first Jurassic Park movie. ("When you receive your results, boot up your Linux monitor and . . .") As for math, the methods for teaching it do change. It can be disastrous for students to get used to solving problems one way, only to get into college and find out their professors expect them to use a method they have never learned before, while the richer kids have known it since fifth grade.

Business books can't be more than 3 years old, because that field changes so often.

The group frequently gets donations of old, moldering nonfiction that recognizes East and West Germany as two different countries or warns us about Satanic ritual abuse or things like that. Even if the donors know it's obsolete, they figure there must be someone in the world that would be interested in owning this book for historical/nostalgic reasons. There might, but those few aren't likely to walk into our midsize town's library. There is a member on staff who checks online for trends among book collectors, but the group does get a lot of stuff that the library cannot reasonably expect to resell.

And ripped, broken, rotted books: forget it. It makes the other books around it look unappealing.

Bottom line: not every book is reusable material, and textbooks especially are at the bottom of the pile for that. Nowadays, teachers are very proactive about searching for secondhand recent and relevant textbooks, and those get snapped up quickly enough. The rest would just be taking up space.