Can anyone point me to a good introduction to coding against the paypal API?
I would suggest you start by downloading the SDK:
https://www.paypal.com/IntegrationCenter/ic_sdk-resource.html
The SDK includes the following:
- Client libraries that call PayPal APIs
- API documentation for SDK components
- Sample code for Website Payments Pro and various administrative APIs
- Testing console that can verify connectivity to PayPal and submit API calls
You may also want to take a look at Encore Systems .NET* Class Library for PayPal SOAP API
Found this article by Rick Strahl recently http://www.west-wind.com/presentations/PayPalIntegration/PayPalIntegration.asp.
Have not implemeted anything from it yet, Rick has quite a few articles around the web on ecommerce in aspnet, and he seems to show up everytime I'm searching for it.
I don't know what your needs are, but you might want to consider Google Checkout. Joe Audette was having considerable difficulty integrating PayPal.
I've used Google Checkout and have had great success. Note that you can go much, MUCH deeper with Google Checkout than the sample linked above.
EDIT: I didn't see Joe's updates. Look like he did eventually get it working.