Saw this a bit late, but I think you might be interested in a PayPal IPN (Instant Payment Notification). You will need a website in order to effectively use an IPN if you do not have one already.
Here's an example worth taking note of:
http://youtu.be/6IfwYMI25L8
This video contains a decent summary of how an IPN works and how you can get one set up with ease. If you're familiar with PHP, the code provided is easy to expand upon and implement into your own system, which can be handy if you'd like to store purchase logs or give the client post-payment permissions (e.g allowing them to download your e-book/program).
If you'd like to create your own from scratch, PayPal have resources available to get you started here:
https://developer.paypal.com/webapps/developer/docs/classic/ipn/gs_IPN/