Introduction to PayPal for C# - ASP.NET developers

C#:
http://www.codeproject.com/Articles/42894/Introduction-to-PayPal-for-C-ASP-NET-developers

http://www.dotnetfunda.com/articles/article928-online-payments-using-paypal-integration-with-aspnet.aspx 

http://www.codeproject.com/Articles/19184/Use-of-the-PayPal-payment-system-in-ASP-NET 
http://www.dotnetspider.com/forum/313739-How-integrate-PayPal-.net-applications.aspx 

VB.Net

http://www.nullskull.com/q/10422784/how-to-get-transaction-details-from-paypal.aspx