i think this will come more and more a normal thing to sell capx as construct matures and has a bigger userbase,
the best thing to do is make a good demo to generate interest, prefereble with some semi-good/good graphics and interaction, and even post improvements/finetuning to the engine to keep interest
make a forum page and an arcade post
i would calculate the time spended on it, * your hourly rate / estimated buyers
lowering price should generate more buyers + better demo = more buyers
this does raise some questions, legally, if you capx should get posted
but i guess the same thing can be said for art, if you have no permission to use it, your stealing..
these where just some thoughts to consider, i would definatly go for it