Why did I get the error "A project with an output type of class library cannot be started directly" with the ASP.NET SDK?
This error occurred because the ASP.NET samples included with the PayPal SDK were built as a class library and not an ASP.NET web application. You can't run the sample in debug mode and through it as is.
Create your own ASP.NET web application referencing paypal_base.dll if you want the ability to step through your solution in debug mode.