jrockfl - 2:47 pm on Nov 14, 2012 (gmt 0)
When a customer submits the order, it has a "Gateway Successful" flag.
If the payment goes through successfully, the IPN notifies a page on the web site that sets the flag to true.
Yes, the sandbox is good place to setup test accounts to test the entire transaction process.
Yes, you have to turn on IPN.