I accidentally bought a fake on Poshmark. How do I handle the dispute?

I bought a bag about 3 weeks ago on Poshmark. I paid to have it authenticated and it came back fake. I have already accepted the order. (Dummy move I know). I paid through PayPal using my discover credit card. I opened up a case through Poshmark but if they refuse to help, can I dispute it through my credit card?