[finder83]

It's very possible they don't replace them, but many libraries also have legitimate sells to clear books from inventory to make room. Usually over like a week or three day weekend once or twice a year, the last day having bags of books for $5. All of them have stamps/card holders/stickers designating the library. So those don't necessarily mean they were borrowed and never returned.

[WalterBright]

You may be right, but I wonder why they would remainder a book that was on hold.