At the time, people saw Bitcoin as a tiny market. US-based exchanges were incredibly hard to start. Even Brian and his YC application didn't really believe in his own idea because what he basically applied to YC with was the P2P transfer parts. That was enough of an insight to be the basis for a $100 billion company. No one knew that.
Dalton Caldwell and Michael Seibel talk about where the ideas for Airbnb, Coinbase, and Stripe came from. Then they discuss what you can learn from these founders.
To create Rookies Mistakes we asked YC founders: Is there a simple fact you wish you knew when you started your company or a rookie mistake you wish you could take back?
Apply to Y Combinator: https://www.ycombinator.com/apply/