IPN Listener

 

Reference:

  1. http://stackoverflow.com/questions/11810344/paypal-ipn-bad-request-400-error
  2. https://developer.paypal.com/webapps/developer/applications/ipn_simulator
  3. https://developer.paypal.com/webapps/developer/docs/classic/ipn/integration-guide/IPNIntro/#protocol_and_arch
  4.