To leave the Notification URL blank would imply that I need to DISABLE IPN Messages on PayPal otherwise it doesn't allow me to leave the box empty - must insert a properly formed URL.
So if that is DISABLED how will I get IPN from PayPal? Does the settings on nopCommerce somehow do that ?
IPN info is sent together as part of the request too, so you may just turn it off in settings. As far as I know, the settings you set in PayPal is just the Default Settings, it'll get overriden depending on the requests. :)