Alibris (https://www.alibris.com/) is a great alternative. I do basically all my book buying there.

If anyone’s concerned about the condition of used books, I’ve found their condition descriptions (https://www.alibris.com/popups/glossary/book-condition) to be reliable (indeed, books are often in better condition than indicated - I’ll happily order anything ‘fair’ or above).

Isn't AbeBooks for collecting old books? That's what I use it for. Abebooks and eBay. lots of out of print vintage niche books that way, like early JA->EN translations of novellas.

biblio.org is a good alternative where I am (although personally I don't see the problem with having either the print-on-demand books or buying used from Amazon as an option).

It is annoying, but bookshop.org is a good alternative to both Abe and Amazon, presenting a single shop front to lots of bookshops.