It doesn't say that anywhere in my DMG. In fact it says pretty much exactly the opposite of that:
So maybe you can tell us what page this hypothetical rule is on in the DMG.Originally Posted by DMG
Or perhaps you can tell us the hypothetical rule that allows PCs to know an item exists and therefore look for it in the city?
Absent that, I'm going to have to stick with "The characters know what items can be created/wished/bought because knowing what items can be created/wished/bought is just something that is assumed everyone relevant knows."