It will never happen. PayPal is in the business to protect their customers.
Reasonable doubt will be investigated.
The problem with crypto currencies -- there is no 'physical' product... and such, no acceptable proof of delivery.
Perhaps in the future PayPal would create a 'PayPal' coin -- not mine-able, but transferable.
If they have control, they have the proof of a 'transaction'.
What about a website/exchange where users can make buy or sell listings and if you want to buy, you send gift money to the website and your account is credited $$. If you want to sell you send the coins to the websites wallet and the exchange is done when both parties agree and complete.
Anyone a developer, I think that would work.