This... might not be possible, or at least not practical, with Unity, I have no experience with the engine from the coding side of things. But just in case, a small request: it would be very helpful if we could see how many of each item in a shop we currently owned. Perhaps when you mouse-over an item, the info box could also list the number of them in your inventory? Particularly when I'm browsing the furniture store it would be nice to be able to see which of the available stock I needed to top up on.
If nothing else, just adding a few more tabs in our inventory Sack next to Consumables and Key Items that listed these things would at least allow us to manually check more easily.