Ordinary language is all right.
One could divide humanity into two classes:
those who master a metaphor, and those who hold by a formula.
Those with a bent for both are too few, they do not comprise a class.
The interlocutor's question about the shopkeeper is a skeptical one: 'But how does he know…?'.
Wittgenstein's response stresses that, or how, the shopkeeper acts, as if to say, this is not a matter of knowing, but of just doing. But it also contains a curious clause: 'I assume that he acts', Wittgenstein says, 'as I have described'. But could it be any other way? Taking the stress off 'acts' for the moment, what he is saying seems to affirm the natural—usual, obvious—connection between what the shopkeeper does, and what Wittgenstein says the shopkeeper does—between the shopkeeper's behavior and Wittgenstein's description of it. Wittgenstein affirms the ordinary adequacy of our words to their objects.
Could it be any other way?
In Investigations §1d, the difference—if not disagreement—between the interlocutor and Wittgenstein concerns what is essential.
Sometimes I just get this urge to go home and read Heidegger.
A special sort of imposition, playing a role and expecting the one you're playing for to play one in return.