I am a theme developer and I am looking for a way to build them and get paid for them at the same time. I know I want my cake and eat them too but I am struggling and it seems like a good way to make money.
Sorry but I've not heard of anyone doing this for themes or apps. Even not wanting money while working on the theme people would be reluctant to purchase theme without seeing it. There are alot of theme builders now that do it just for fun and to provide a service that post their themes on sites for free. It make take you doing that just to get established and get people to give positive reviews to draw interest in your work.