File read/download permissions and selling pdfs

IESIS - Institution of Engineers and Shipbuilders in Scotland - are seeking to create a digital library of several hundred pdfs and potentially other file types.

Most of the pdfs are to only be read and download accessible to members of IESIS, with a small number of sample pdfs made available to everyone. There would also potentially be a feature for non-members to pay for individual pdfs.

Does Omeka have a permissions system to limit file access in this way? Is it allowed, under the terms of the licence, to charge for pdfs?

Omeka, and the underlying Zend Framework, have permissions systems, but to my knowledge none that build out a check on membership or payment system.

A plugin for Omeka for that sounds feasible, but doesn't currently exist (again, so far as I know)

My understanding -- not a lawyer -- of the license for using Omeka puts no restrictions on that kind of site. The only licenses are about the code itself, not about the sites that you build with it.