Here are the exact chargeback rules of PayPal. What I do not know is, even we can prove that a certain transactions of Bitcoins are succesfully done, PayPal might not reverse the charegeback.
BTC being a digital item, good luck with that. Look into Paypal's digital items policy.
https://www.paypal.com/us/webapps/mpp/ua/useragreement-fullIneligible items and transactions
Your sale is not eligible for coverage under PayPal’s Seller Protection program if:
It involves intangible, non-physical, items, including digital goods, and services. Digital goods are delivered and used in an electronic format, like a song delivered online or through a mobile application.
The buyer claims (either with us or their card issuer) that the item you sent isn’t what was ordered (referred to as a “Significantly Not as Described” claim).
It involves an item that PayPal determines, in its sole discretion, is a counterfeit item.
It involves an item that you deliver in person, including in connection with a payment made in your store location.
It involves sales that are not processed either through a buyer’s PayPal account or a guest checkout transaction. For example, if the sale was made using the PayPal Payments Pro/VT product, PayPal business payments or using PayPal Here, then it is not eligible for coverage.
It involves items equivalent to cash including gift cards.