Sell Back Your Book Scam artist company offered high payment on exact ISBN and book and free shipping, and then claims I entered the wrong ISBN! Internet

I was looking to sell my textbooks, and I found Sell Back Your Books. It is literally impossible to enter the wrong ISBN and not know it on the website for SellBackYourBooks.com because once you enter the ISBN an image of your book's cover pops up and you can confirm that it is the correct book. I confirmed all three of the books that I sent to Sell Back Your Books. After I was quoted over 90 dollars for all three of my books and sent the books in using their free shipping, they sent me an email claiming that the two most expensive books had the wrong ISBN and were quoted at significantly lower prices! First of all that's impossible, it's an egregious lie and it's deceptive business practice. If I'm not satisfied with their new quotes they also want me to pay 4 dollars shipping to have them returned, and I'm not doing that! My quote went from 90+ to 20, for the correct books! They may not do it all the time, but on the books worth a lot of money they must offer extremely high quotes and then lie to you about sending the wrong books so that they can low ball you on the books!! I am reporting them to the BBB as well because their customer service is a LYING and UNETHICAL center, and they also don't have a full time staff apparently so there is no convenient way to call them. 

SO FRUSTRATED.