Just a thought.
For sales of complete C3 games from the Scirra store, include a month's subscription for just that game.
So basically who ever you sell it to, be it another C3 developer, or game publisher, they can open only that game, edit, and export it for a short time included with that game's license.
This is to alleviate the issues of selling selling C3 source files without a way for someone who is not a regular subscriber.
Something we have run up against with C2.