Lean Startup theory can be pain in the ass for many kind of startups. I think it's wrong that Lean Startup doesn't encourage sales from the very beggining. Sales are the best validation of your idea, not feedback from customers who doesn't pay for your MVP anyways. Build a product for people who pay for it - that's my advice. It's not universal eighter, but more than 50% of startups can start sales before coding anything.