you could sell content subscriptions, but not the CMS itself, plus any modules you develop need to be posted for public consumption.

say you research information on "Topic Z" you could give access to paying subscribers... they are buying the content not the CMS.

you could develop a module and try to sell it... and you may, but if it is trully usefull somebody smarter will re-engineer it and give it away.

someone may have $ but no skills or time, and pay you to develop "Module Z" to their specs. Thats fine but the module is public.

think of this as a community in which you can trade modules at no cost... but if you score some $ with your site... you should remeber the tip-jar on this site, and help the community continue.

As I understand it anyway...