This is not really a literature question, more like a book business question. A few weeks ago I was shocked to learn that when I donate a book to my local public library (in the USA), the book is very unlikely to find its way to the library's shelves. Most donated books are sold to the public at book sales held once every few moths.
This is incomprehensible to me. Why do libraries do that? If the library already has a copy of a donated book, why not trade it to another library for another book that is not, for the mutual benefit of both libraries? Here in the Boston area, there are probably fifty academic and public libraries within a ten mile radius, yet each of these libraries offers tens of thousands of books for sale every year.
Can a librarian or someone familiar with library management explain this to me?