Hello 'jorgcanti',
You will want to return them to the seller from whom you bought the books, and that means first use either the Ask a question link at the bottom of the listing or click on the seller's ID and choose the word Contact from over on the right.
The seller will tell you how she would like you to proceed. In the meantime, if you wish to see ebay's official page on how to do a return:
http://pages.ebay.ca/help/buy/return-item.html