Well if the person hits cancel and then goes back and pays via paypal then they do not get forwarded back to the site so things are approved. Same thing user goes to paypal and decides to say check his balance etc or go to some other page in paypal and comes back to the paypal screen the form return url is now empty
A user must upload an ad pay via paypal immediately hit submit and they are forwarded back to your site. The ad is approved when paypal forwards them back to the site with a special code and action that says paid. If they do not make it back to the site with this then you may get paid but the ad is not approved.