amazon is a easy way to sell used books or craigslist...maybe letgo app.
I've had luck with posting books separately on Amazon. It can be painstaking with a lot of books but they do sell if priced right.
Recommending Amazon as well: some books Amazon will buy from you and you can get Amazon gift cards (if you want that, not sure if you want cash or if gift card money is fine) while the others (or all if you choose) you can sell yourself in the same way the other small time used sellers do on Amazon.
Give /r/books4sale a try
Try bookfinder.com/buyback it checks a lot of sites