The trouble is although you get billed for the book, the library does not replace it. I've had things on hold get cancelled with a note about "no longer available".
I've bought a lot of books from the thrift store that had library stamps in them.
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.
I've bought a lot of books from the thrift store that had library stamps in them.