Our library has 2 book sales a year (working on getting down to one a year) and a very small sale around Christmas that consists of "like new" books that people can purchase as gifts. We always have used paperbacks that we have discarded or that people have donated for sale as well.
We make it clear to people who donate that what we cannot use in our collection we will put in the sale.
I have donated many books to my library and it makes me smile every time I see someone check them out.