Where can you sell books to get the best price?

I have many books that I've bought full-price from Books a Million and Barnes and Noble brand new, thinking they would be good, only to realize they weren't good and that I definitely didn't want to keep them!
I know I won't be able to get my full money back, but where can I sell them to get the most of my money back?
Is there any kind of store that would buy them, and what are some good online sites to use?
(I'm not very tech savy, and don't understand how some websites work, and how to figure out shipping costs and all that).
I'd really like a simple, straightforward and trustworthy website! Thanks!

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