Marketpress Question regarding external links

I am trying to set up a restricted membership site and am struggling with how to make sure that site members can't use the site without me generating revenue when they list a product on Marketpress.

With their ability to use external links why would they use the built in marketpress to add products option? If they use an external link and the product is entered into the global marketplace these products are not earning me any percentage of their sales.

Conceivably my global marketplace could end up filled with a ton of external links and I would earn nothing from those sales?

Is there a way to turn this option off and if there isn't why not?

I have tried to set this up then as a paid membership site with different levels of access and find that hard to do because most clients will assume if they are paying a monthly fee they will not also have to pay a percentage of their sales. Or do you guys see that as still a fair option?

Is it the normal for someone to have to pay both a monthly fee and a percentage of their sales and if they are paying a monthly fee then maybe that covers my loss of revenue from their external marketpress links????

Thanks for any ideas.

Cheri Faline