Paypal IPN to update 'units sold' on website

Sorry for the very basic nature of the question, but I've been searching for two days and I'm still not sure how to do this.

I've got a paypal cart/store set up. How do I leverage either paypal IPN or a third party service to identify one specific item of the three items I have for sale, and send me a notification that a copy has been purchased, and then update a counter on my original website?

Again, just suggestions or a guideline would be greatly appreciated - even to point me in the right direction. Thank you.

Solution

A basic guideline for you:

have a sandbox account (I assume you already do)
create there a seller and buyer account
set IPN setting for seller account to your testing URL (where you will be checking for certain data returned)
for the URL mentioned above create a php file with IPN validator
and go here https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_IPNandPDTVariables#id08CTB0S055Z to check variables you will be searching for.

I assume you would need to pick num_cart_items and loop throug possible item_nameX variables returned.

You don't need to code anything to validate IPN, use some of these classes: http://www.micahcarrick.com/php-paypal-ipn-integration-class.html http://www.phpclasses.org/package/2249-PHP-Process-Paypal-payment-interactions.html