If you want to use PayPal Sandbox with Web Form Builder, there is a small change that you will need to make to one of the PHP scripts.  Please note, this does not work if your form is hosted on S-Drive at this time.

Step 1:

Edit the file fbapp/php/checkoutcontroller.cls.php

Step 2:

Now comment out line Line 14 and uncomment Line 15.

Line 14 //define( 'PAYPALWPS_URL', 'https://www.paypal.com/cgi-bin/webscr' );
Line 15 define( 'PAYPALWPS_URL', 'https://www.sandbox.paypal.com/cgi-bin/webscr' );

That is it.  Your form will now use PayPal Sandbox.  When you are done testing, make sure to reverse the changes. ;-)