Where is the best place to sell a new book.?

I received Eclipse and Breaking Dawn as a Christmas present and I do not need these books since I have them already because I won them in a contest. I tried to take them back to the store where they were purchased and they would not take them back because they said it was a 24 hr return policy. Does anyone have a good idea where i could get the most money back for these books because it is obvious that these books have never been opened or read? Any help would be greatly appreciated.