When you eat another Jew's food, you are entitled to rely on him for the kashrus of the food. By that reasoning, if he opens a restaurant, and you know him to be Yarei Shamayim, you shouldn't need any external certifying authority to eat there.
But I've been told that as soon as someone opens an eatery for profit, one can no longer rely on his personal Yiras Shamayim, but you need a 3rd party to certify his kashrus.
Is that true? If so, please quote sources.