I would like Amazon to, effectively, offer that demon as an option, to guarantee the provenance of an order's items. Of course, it might work better to filter commingled items out at search time, if that's possible, or put a big warning banner up on the product or order page when you might get your item from another seller's inventory.
The difference between 1 fungible pile of a good spread out everywhere, and multiple distinct piles is enormous.
It's only a guess, but I would think the added expense wouldn't be that big of a deal. With popular products, they already have lots of units everywhere so it would very rarely be an issue. With other products, it is more likely to be an issue, but still not every time.