As far as physical books go, I’ve been using thriftbooks.com. They’re used but you can choose what condition you want to buy, which will affect the price.
Does anyone else have any other website recommendations?
I used to live near a public library that had a really big free shelf where they gave away extra copies of books they had. Those were the best years of my reading life.
Now I have less time and don't live near a library anymore.
I like those shelves with free books! In the town I live now they are actually quite common, one can find tiny or not so tiny places on every other street, typically organized not by libraries but people.
I own (too?) many books and I'm trying not to buy (many?) more for some time already. I tend to borrow books from libraries as much as possible, and take advantage of the fact that some will take requests from customers to buy books they don't have to add to their collections. When that isn't an option, I try to get them second hand, as well.
You list the books have and want to get rid of, and if/when someone else wants them, you pay to ship them, and then get a 'credit' for a book you can get from someone else for free. You can set up a wish list, so whenever something comes available you can claim it. Been using it for years with great success.