Oh ok so without this anyone that added a paid resource would say have to take payment by PayPal then manage sending files etc from their end via email or something?
If so how do they then manage updates. I notice with some adon I have here that work similar to this. And when an update is available I get an email with the file.
Are they manually sending this update file to their list of customers?
You can setup User Upgrades for the products then use the Resource Manager permissions to limit who can download the products until they have paid for the upgrade. Depending on how many products you have this can get quite messy though.
With that said I'll +1 the XenProduct Manager for ease of use and functionality for downloadable products!