So it turns on PayPal's TOS vs common sense/public policy. Should PayPal be allowed to operate TOS that facilitate fraud?
It is hard to see exactly what the legal argument against PayPal would be. Their TOS are not illegal. The argument that they are aiding and abetting a fraud sounds interesting, but it seems like they have a good defense in claiming that the fact this type of fraud can occur is precisely the reason they don't allow the sale of BTC on Paypal.
Certainly there is a case against the the other party who received the BTC, if you can find them.
If proceeding with this suit, I hope you will follow up here. I'm very curious to see what happens with it.