In my state, there's a book chain that bought about 60% of my collection when we culled it down. Not for large amounts of money or anything, but we were happy with what we got.
I have heard of apps you can scan your books for to see if you can sell them online, but seems like lots of trouble. Agree with a few of the methods others have mentioned. But if you have a larger collection of books that will take you a while to go through individually, you'll need to decide if the time waste is going to be worth it to you. For us, it wasn't.
I love books. I have an entire room in my house with floor to ceiling book shelves. My husband is the same way - he worked in the library of his college for goodness sake!
The rest we donated to our neighborhood library as they organize monthly book sales and it would be the best way to help them out and help ourselves out as well. The hassle of trying to ebay or otherwise try to sell off books that a retailer didn't want seems like a lesson in time wasting.
So if they are books you sort of kind of don't want, find a reseller in your area and take them all in. Any they don't take, figure there's very little market for them and donate them someplace that might make a few bucks off them for charity.