Here in the UK, I often end up getting books from World of Books, who are fairly ethical as for-profit companies go
I have found Biblio is a great way to buy used books affordably, just make sure to search the ISBN of books you want (I use amazon for that usually lol).
Biblio is NOT owned by Amazon unlike many of these "independent" companies that were bought up by Amazon.
Awesome, I love that you can get used books through them!
They are ostensibly focused on finding rare/specific editions or out of print books, but the network of independent book sellers they have is pretty huge so you can find almost any used book you want there, not just niche stuff, it is a great resource!
I have bought many ~$10 and under used paperbacks in good enough condition for me from them.
They’ve been big since before amazon got started and have survived and thrived as an independent company since. It’s the darling of Portland and it shows, every time I’ve been there it’s just packed with people, but I usually buy from their website.
Great list!
There's also bookshop.org for buying books (and ebooks) while supporting a local bookstore of your choosing.
Oh, and libro.fm for audiobooks!
Ok, I've actually used Bookshop.org, so I don't know how I forgot it!
I'll add Libro.fm as well--I'm not an audiobook fan myself, so I tend to forget those. I'm pleasantly surprised to see it's an employee-owned business!
Last Christmas I was having a hard time tracking down a book I wanted to gift. It was out of stock at all the big stores, but I found it at https://www.summerbeambooks.com/, a small bookstore down in Oregon.