Bookbyte.com Withholding money promised to me from products I sold to them; claiming I sent wrong product Salem, Oregon

Bookbyte.com bought three textbooks from me on January 3, 2012. I shipped those books out on January 4, 2012 at my local USPS location. In the email confirmation of the books they bought, it said bookbyte.com would issue the payment owed to me when they received the books. On February 21, 2012 I sent an email to the customer service address telling them it's been almost two months since I shipped the books and I still haven't received my payment. Nicole Westby, a representative, emailed me back stating there were electrical cables inside of the box instead of the three books. They said they were able to track the order and know I sent the electrical cables by the barcode on the shipping label. The shipping label didn't have a barcode on it because it was a basic media mail label that anyone could print out from the website.

 I've been emailing the customer service email address asking for my clarification, why they didn't contact me about where the books were, and why they didn't question the electrical cables. They are withholding the money owed to me by claiming it replaced the books with electrical cables. Bookbyte.com has gained my textbooks at no cost to them by not issuing the payment owed to me.