at the library

Bookfinder.com which led me to checking eBay on a regular basis.

Libgen, but I have a kobo e-ink reader and 90% of what I read is nonfiction

I buy secondhand online when I can from non Amazon websites, there’s several of them. I’m sure they’re probably flawed in their way too but it gives those books new life and you get them cheaper

annas-archive.org

Bookshop.org is the first place I look.

I have had a good experience buying second hand online from https://www.betterworldbooks.com/ and https://www.thriftbooks.com/, they're both good when it comes to looking for used leftist theory books. Other than that I typically just check out my local bookstores or libraries to see if they have any used copies of something that I'm looking for, it can be really hit or miss though.

Online classifieds (in Norway this means Finn.no) and also the to-be-retired shelf outside my local library. If you're in Norway there's also arbant.no

AK Press. They're more anarchist/left-com, but they've got some Marxist books.

For those who find this post later; biblio seems pretty solid besides being a non-worker owned business. It is what it is, we live in a society. They are certified as a "living wage business" and use a triple bottom line business structure which seems better than most at least. (Uncertain as to exactly how much these benefit their workers but it seems good)

They are partnered with the indigenous literacy foundation (you can round up, they don't donate themselves 🙄) and HAD a non profit that built libraries in bolivia which is pretty cool.

Its certainly better than amazon so i'll take what i can get

The library for most things, and the books that I keep I've mostly gotten from friends and family (or garage sales and the like for the rare stuff). I currently have filled all 4 of my bookshelves, and books now cover most surfaces.