SDKs https://www.x.com/developers/paypal/documentation-tools/paypal-sdk-index
Main Page: https://www.x.com/developers/paypal
PayPal Labs integration wizard - not sure how good code this tool produces, but it's worth checking out https://www.paypal-labs.com/integrationwizard/ecpaypal/main.php
Getting started with Express Checkout https://www.x.com/developers/paypal/documentation-tools/express-checkout/integration-guide/ECGettingStarted
A very good overview of the PayPal API options http://www.codeproject.com/Articles/42894/Introduction-to-PayPal-for-C-ASP-NET-developers
Good tutorial using the web services API http://blog.effectlabs.com/post/2011/11/07/Paypal-Express-Checkout-with-C-using-Paypal-web-services.aspx
How to view SOAP messages http://blog.encoresystems.net/articles/how-to-capture-soap-envelopes-when-consuming-a-web-service.aspx
Some more paypal SDKs from their Australia site https://cms.paypal.com/au/cgi-bin/?cmd=_render-content&content_ID=developer/library_download_sdks
A very simple tutorial that illustrates the basic points, although it does not go to the advanced functionality, but it is useful if we want to get a first feel quickly http://www.dotnetfunda.com/articles/article928-online-payments-using-paypal-integration-with-aspnet.aspx
