You can try looking up your books on this site:
http://bookscouter.com/
It searches up to 44 of the most popular book buying sites on the internet and it tells you which site is offering what price for your book. Some sites do require a minimum.
Book Scouter also has reviews for many of the sites so you can decide for yourself if it is reputable.
Also, some book trading site include:
www.bookmooch.com
www.swap.com (small fee for trading)
www.paperbackswap.com
There are others. With these, you pay for shipping the books to others and the books you request are free to you.
I forgot to mention that Amazon now buys back books, too. But, you get Amazon gift cards. The good things is that Amazon sells tons of stuff, so you could use the gift cards to buy gifts, or even groceries! (It says textbooks, but I have sold them no-textbooks, too.)
http://www.amazon.com/Textbooks-Trad...ode=2205237011