You'll want to open up a PayPal dispute immediately. Of course, the seller is probably going to claim the book has been tampered with since he sold it to you. He'll argue that it's no longer in the PGX holder, so it's not the same book. If the seller were honest and reputable, not saying he's not, he'd take the high road and refund you your money then he would be the one to go after PGX and whoever sold it to him.
Cases like this are why you use PayPal. If PayPal doesn't intervene, then you have no choice but to contact your credit card issuer. PayPal can and will side with the credit card issuer. What I mean by this is if PayPal denies your claim then you file a dispute with your bank, and they approve the claim, PayPal by law and their underwriting would have to reverse the dispute to your favor.
I know you are probably pretty pissed, and rightfully so, but the good news is you put this on a credit card and paid through PayPal so as long as you are inside their window for filing a dispute, you will be ok.