In the sandbox area you create 2 accounts. Account 1 would be the one that gets payed. Make it a business account. Account 2 is the normal user that pays.
In order to use the sandbox when testing you do need to be logged into the developer central. (main place you login to).
Instead of posting your data to Code:
https://www.paypal.com/cgi-bin/webscr
you post it to
Code:
https://www.sandbox.paypal.com/cgi-bin/webscr
From there it works like normal paypal except you use these 2 fake accounts you created.