It's not trivial to set up PayPal IPN (Instant Payment Notification). It requires a free Business (Premium) PayPal account, a web hosting account with PHP support, configured to allow creating HTTP socket connections using "fsockopen" function, a MySQL database set up on your web server, a mailing mechanism (for sending the product to the buyer) and a free Gmail account. It may seem like a lot, but everything except the web hosting server (which you probably already have) is free, and once it is fully...