Reading the question "Rich Wizards, Poor Muggles" about wizard money and muggle money, I think I hit upon a contradiction. In the answer to that question, it was stated correctly that:
For the most part, there's very little that members of Wizard society want or need from Muggles. Property, housing, and food really seem to be about the extent of it for the vast majority of Wizard families.
Though most of the Wizards seem to have completely no knowledge of muggle money and how it works. An example of that is Mr. Weasly when he had to go with Harry to the Ministry of Magic by muggle transportation (using the subway), and have in mind that if Mr. Weasly doesn't know of how muggle money works with all the love and interest that he has in them, I think there is very little chance that others have better knowledge of it.
Some Wizard families grow their own groceries, though they still need other stuff that has to be bought, like bread or meat etc. And for sure because of Gamp's Law of Elemental Transfiguration we know that food cannot be conjured.
In addition to that, I have to assume that Wizard families buy their food (groceries, meat, etc.) from muggle stores, for which they would need muggle money and of course the knowledge of how they work (currency, etc). (I have never found a statement about butcher or grocery shops, for example, existing only for Wizards, even though in the answer to "Rich Wizards, Poor Muggles" the answerer assumes that there is such a place in Diagon Alley for which there is no evidence of existence though.)
These two facts contradict each other and confuse me. Is there a book mention that I am missing here or, if not, maybe from other sources (Pottermore or J.K.R interview) which states the existence of shops (to buy groceries, meat etc.) existing only for the Wizard community? Or else how is it possible that Wizards do their food shopping from muggle stores when they don't even know how muggle money works?