There is a good article for Integrating PayPal into E-CommerceApplications with ASP.NET on DevX by Rick Strahl.
It starts like this:
If you run a Web shop that uses direct credit card processing and youwant to integrate PayPal, you’ll find that using PayPal as a processingservice is not as straightforward as using a payment gateway. In thisarticle, I’ll describe how you can minimize the external PayPalinteraction and work the PayPal payment into your order processingworkflow to provide a seamless interface using ASP.NET and C#.
Read full article at: http://www.devx.com/codemag/Article/29342