Angry customer

I recently listed a book and it was bought by Textbook trader. Courier company came to pick-up the book and I waited for payment but I never got it. After 7 days of waiting, I then wrote them an email asking what was the hold-up. It is then that they told me that I gave them the wrong textbook. I requested a picture of the book and it was the very same same that they bought from me. Now, they don't want to return my book unless I pay R200 couriers fees.