This is at its core tremendous advice... But let me temper it: don't sell, validate. In many applications sale risk is cheap enough (customers forgiving enough)

This is at its core tremendous advice... But let me temper it: don't sell, validate. In many applications sale risk is cheap enough (customers forgiving enough) that sales-as-validation is ok. Sometimes it sets up really bad expectations, though. It's easy enough to validate without selling: just don't take people's money. It's less valuable than sales since you can't be sure people are making accurate financial decisions until you have their money, but it's both dramatically better than sitting in isolation coding and carries very low social risk.