I have used a variety of online book stores and buy back places before however this has been the most dishonest experience I have encountered by a company that supposedly has fairly high ratings. Perhaps there was a dishonest employee working the day they received my book however I'm the one that got cheated in this case. I recommend reviewing their yelp and reddit forum for feedback versus some of these other sites to get more candid reviews.
This company steals your books and then changes their excuses for why they won't pay you. I have evidence of the book I sent with correct ISBN number however PiggyBook/Discover Books first claimed that they didn't receive anything in the package from the USPS. When pressed, they then said it was actually an incorrect item. When pressed again for them to provide me with evidence of how the book magically changed in transit so I could submit a USPS claim, they said they couldn't provide me with any evidence as they trash everything at the end of the day. This company is incredibly shady and I would not recommend at all.
This company is also known as Piggy Book so please avoid if you don't want them to steal your items and then make bogus excuses without ever providing any evidence.