However, there is perhaps a misunderstanding. I don't want to build the mirror of a paid version of rubygems.com. I simply use the gem workflow mechanism (which is robust and easy to use) in order to sell modules BUT in the context of a project (open source or not).
Besides, in php, there already are open source projects successfully working this way, I'm thinking about prestashop for instance. I also learned that some companies sells redhat and debian packages. Why not in ruby ?
One last thing, I'm not defensive but your first post was not constructive enough. You talked about a similar experience you had before (I quote: "... as experience has taught me that buying plugins for an open-source codebase is not an ideal solution for my customers"). Could you tell more about that ? That's the kind of answer I was expecting here. Thanks anyway :-)