The thing about item boxes - you can't put there anything that is not an "item". Fish that was caught in the river is not an item, and in the village where Miria lived - that's exactly the fish that was on sale.
The cheapest coins are also not an items, they were produced by the humans, because the cheapest "item-based" denomination was still too large, and you needed a smaller change. So they don't go into the "wallet" skill (or whatever it is called).
Fish that they bought was the item-drop from the dungeon - obviously, in a dungeon city it will be circulated. Once you try to "use" it in any way, it transforms into just a physical objects and not an item anymore. All items are always the same and uniform, but you can change its form after "item"-type is removed.