Envato / PayPal already do allow payment by Credit Card. So that's certainly not the issue.
So even though PayPal allows payment by Credit Card directly (without having / creating an actual PayPal account), you're still unable to purchase this way?
Have you tried the alternate payment deposit payment processor Skrill / Moneybookers? Does that fail too?
If both fail, you may wish to check with your bank to see if it's not being declined for whatever reason or use a different card.