Sometimes, people are willing to pay a small amount of money for a book, but not retail price.
A place you can buy a book for a small amount of money (usually smaller than second-hand book stores) is the library book sale.
What if you could sign up to be automatically notified when a book you're interested in is added to the library book sale, so you can get the books you're interested in owning at an extremely affordable price?
Depending on how their computer system works, you could either get a notification when a book is added to book sale inventory, or when it is removed from the catalogue because it's being weeded from the collection.
Also depending on how the system works, maybe they could put books aside for people who signed up for notifications rather than putting them directly into the general book sale. (I feel like putting a book aside for the person who requested it is already within the library's skill set).
The book sale could charge more for this priority - maybe $2 or $3 or a book that would normally be $1 - thereby raising more money for the library while still being a fantastic deal on a used book. Win-win!