There's an interesting ecosystem around weeding. Many libraries send weeded books to Better World Books, which is a major used bookseller. If BWB don't want the books, then they pass them along to organizations like the Internet Archive, which scans them & then loans 1 electronic copy for each physical copy. Better World Books averages 1 container of incoming books per day.
I've only dealy with them as a customer, they're not good at packaging books for posting but if most of their stock is old penny-books then presumably they're trying to make margin on the Amazon postal charge.