using sandbox in paypal
I am just working on that issue. For pp sandbox you must use the sandbox credentials. This means that you mus create an account for a buyer and/or an account for a merchant or website payments pro.
The buyer uses an email (test user account) a pw and a card nr. The merchant and the website payments pro needs email (test account) , pw, api username, api pw, signature, cars nr and bank account.
When you go live you can also use the sandbox for testing. In real mode you must change them to the real accounts.
I hope I was helpful