Test credit card numbers for use with PayPal sandbox
In case anyone else comes across this in a search for an answer...
The test numbers listed in various places no longer work in the Sandbox. PayPal have the same checks in place now so that a card cannot be linked to more than one account.
Go here and get a number generated. Use any expiry date and CVV
https://ppmts.custhelp.com/app/answers/detail/a_id/750/
It's worked every time for me so far...
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.