Aaron D. Campbell

This plugins gives you a great framework to use for integrating with PayPal. 
It stores both live and sandbox API credentials and allows you to switch back 
and forth easily. All NVP API calls are passed through the framework and 
default values such as API version, API credentials, and even currency code are 
added to the request based on settings from the admin panel.
It also has a built in IPN listener that validates messages as coming from 
PayPal then throws WordPress actions based on messages received. For example it 
throws “paypal-recurring_payment_profile_cancel” when someone cancels a 
recurring payment that they had set up with you. It passes along all the info 
that PayPal sent to the action, so it’s simple to create other plugins that use 
this one.
Requires PHP5.
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PayPal Framework

PayPal integration framework and admin interface as well as IPN listener.

PayPal Framework vs Payment forms, Buy now buttons, and Invoicing System | GetPaid