You would use your API credentials from your business account in the API call. As for the buyer, you would not be paying with a PayPal account if you are using the DoDirectPayment API. You would be using a credit card. You should be able to use any of the card numbers below to test with. I juse use a Mod 10 check to generate the card numbers. You can just use any valid future date for the exp date, and use 123 for the card security code.
Visa 4539261585419872 4532063588063153 4024007125711126 4916126222581496 4916335290325039 4929693226575720 4916573003035640 4485204470227566 4024007102338976 4024007148183980