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).

I love alibris

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.

Betterworldbooks.com is my go to. They usually have everything for decent prices. And you can buy used or new for most things.

I checked all of the sites above in the comment tree here and the used books are either 60% of the price of new or 150% the price of new, but used (not including shipping on the latter, oof) so this may be where buying books used online is now awful. Gone are the days you can pickup a book for a few bucks including shipping from USPS and it'd arrive in pretty good to excellent condition I fear