This is kind of a niche thing, but if people don't use these websites then they'll definitely die out. These are specifically websites for trading books, not selling them.
Discuss: On Dreamwidth
Link: http://bookmooch.com/
Worldwide. Free to use and to trade, just pay for the shipping of books you're sending out. Get points for listing books and then sending them to requestors. Use points to request your own books from people. International trades require/give more points. I haven't used it since 2021 but I still have points! Huzzah!
Link: https://www.paperbackswap.com/index.php
US only. List books you want to trade, then people request them and you send them out– then you can request books from other people. Charges: each book you request is $0.49 (a “swap fee”) and 1 book credit (you get these by listing books and then swapping them). More on how it works here. They do have upgraded memberships where you get more benefits (here), including no swap fees and other stuff, from $12+/year.
Link: https://www.reddit.com/r/bookexchange/
Worldwide. Reddit forum for trading books! Never used this myself but glad to see it pop up. Unfortunately seems to be mostly people wanting specific $$$ editions of books…
Link: https://littlefreelibrary.org/
Worldwide. Neighborhood book exchanges, usually hosted in front of somebody's house but I've seen them in cafes and even train stations.
Link: https://www.bookcrossing.com/
Worldwide. “Release” books into the wild to share them, or drop them off at designated bookshelves in public areas (including Little Free Libraries). Some direct trades in the forum (not very active). People log the book's movements with a tracking code; the idea is to keep the book moving around the world. I wish this one was more popular because it's really fun!