Uh? What about costs related to payment processing, time lost, and the fact that if you don't have a big stock maybe you even made a new order for new books when they shouldn't?

There are so many reasons why this kind of behavior is toxic... If the intention was to keep the books, but for some reason they decided to return it, that's fine. Even buying thinking you might end up having to return it is still fine, mostly. However, buying with the sole purpose of lending for free, especially from a small business, is awful.

Geez. It seems you've no idea how a business is run.