[Amezarak]

Libraries are already flooded. Libraries regularly send older books to be pulped, due to the fact they often contain outdated perspectives. They also look at last-checkout. There's a huge recency bias in anything except national archival and university research libraries. And donations are not put on the shelves, they go into the booksale pile and then the pulping pile. Collections are almost always only added to via librarian purchases, procuring new books at their discretion and patron request.

A few put them up on third-party bookseller sites for a time, and I've been able to get a lot of rare and notable works for comparatively little money. Actually, a lot of them come from university libraries, so maybe they're not as much of an exception as I think.