As writers, I'm sure most of us here are also avid readers with way too many books on our hands. I have two fulls shelves at home and two whole boxes here at school. The problem comes when we decide to part with some of our beloved texts, books we've collected over the years, perhaps duplicates from gifts. There are TONS of options for selling and donating books.
As an avid supporter of used bookstores, specifically one in my hometown where everyone knows who I am and the words "student discount" magically get me 10% off. I usually take my books there first and get store credit. If they don't take some of them because they have too many copies already or it won't sell, I donate. I also love getting store credit because I end up getting more money that way that goes right back to my shelves. That way, the money stays in the community instead of being spent on gas or a pack of cigarettes. Books are way better.
There's always the amazon option, especially for text books if you're in college or taking classes. Someone will snag it come next semester.
I find tons of great reads a Goodwill. It's a great place to donate books because they are sold for so cheap that people with lower incomes can enjoy your favorite books without spending a fortune. (I recently picked up the complete works of Robert Frost in hardcover for $1.50!)
Another great place to donate books is betterworldbooks.com. They collect donations all over (even on my college campus) and proceeds from sales on their website goes to literacy organizations all over the world. They also have amazing deals on books, especially the 5 for $15 section on their website. It's fantastic and definitely worth a look, even if you're not donating books, Betterworldbooks is a great place to add to your collection. It's for charity!!