Good idea, huge profit potential there but you're artificially restricting your market hugely by restricting to BTC. While I admire the principle, in practice you're better off opening it up to other currencies and offering BTC as an option. Having said that, the development & successful marketing will be a feat in itself. Hope you like SEO!
Seems an odd restriction since captchas should prevent bots, and the 5 posts min seems like it would only encourage spam rather than stop it. Oh well, I don't make the rules.