A bit late in the game but just in case it helps anyone.
If you are testing using the Sandbox and on the payment page you want to test payments NOT using a PayPal account but using the “Pay with Debit or Credit Card option” (i.e. when a regular Joe/Jane, NOT PayPal users, want to buy your stuff) and want to save yourself some time: just go to a site like http://www.getcreditcardnumbers.com/ and get numbers from there. You can use any Expiry date (in the future) and any numeric CCV (123 works).
The “test credit card numbers” in the PayPal documentation are just another brick in their infuriating wall of convoluted stuff.
I got the url above from PayPal’s tech support.
Tested using a simple Hosted button and IPN. Good luck.