@Mathias My library used to sell 99% of the donated books and media in quarterly used book sales. The money went toward various things the library needed money for.
@Mathias The head librarian explained to me there were usually some books & DVDs they received a TON of and it didn't make sense to add them to the collection. If a book was a bestseller you could bet a lot of them would be donated after being read. People were also eager to get rid of magazines and DVDs. They would usually sell the books & DVDs for $1 and the magazines for 10¢ each.