If you have both RM and [bd] paygates installed, then you can set for each RM category to be able to upload commercial files:
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When You add a commercial file, you can set price for it, then users should pay first, to be able to download it:
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Now, there is tab titled "Buyers" where you can see who paid and purchased that Resource:
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Now, imagine for any reason you want to let some users to be able to download it, without paying money. What you can do? "Add Buyer" is what you can use in these cases. Click on "Add Buyer" and then write the username you want, and then click "Save Changes". Then you can see that user is added to the list of buyers, and he/she will be able to download that resource without paying:
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You can also delete anyone, from the list of buyers.