Hello,

Say I wrote some serial key generating code, how do I implement it so that after someone buys a program from my website with Paypal, the website generates the serial and shows it or emails it to them?

Thanks.

The paypal IPN system already has provisions for this. Read the API for more information (don't have the direct link on me right now)

The Paypal developer area will likely be of use to you: https://developer.paypal.com

It contains a testing environment amongst other things, and should hopefully have everything you need :)