I disagree, as an engineer building is too tempting and will get you in trouble. Instead just start selling. Keep the building to an absolute minimum until you've sold something.
Nope I still disagree. Having sold something that doesn't exist is a good position to be in. Then you just make it real fast (we're good at making stuff!)
In the worst case you can't make it fast enough and you lose the customer, but you've at least proven there's a market for the idea. There will be other customers.
that sounds good in vacuum but in real life if you fail one customer it can have ripple effect for future. This is Why i would suggest always to build MVP and start selling from that point. IT is easier to sell something new to existing client rather than sell blind and fail.