Maybe try contacting itch.io support? What does it say when you try to buy one of his plugins?
This is the message I get: 'Failed to create PayPal payment (422)'
This is what i found on the web:
Error: 422 Unprocessable Entity
This error means one of two things:
Your PayPal account must have an adequate balance to cover the payouts it is attempting to process. Unlike personal PayPal accounts that withdraw from a bank or credit card source, you must have a sufficient existing PayPal balance to cover the referrals you are attempting to pay out, or you will receive this error. Also, know that your PayPal sandbox account has a separate balance than your live account, so make sure it is adequately funded as well prior to testing with a sandbox account.
The currency code the funds are being sent in cannot be received by the recipient due to country or account regulations.
I have enough funds in my account I assume there is a problem with his paypal account.