> For inventory tracked with the manufacturer barcode, each seller’s sourced inventory of the same ASIN is stored separately in our fulfillment centers. We can also track the original seller of each unit.
> To fulfill your orders exclusively with your inventory, you can switch to the Amazon barcode at any time (see Changing your barcode setting below).
Source: https://sellercentral.amazon.com/gp/help/external/200141480?...
It seems vendors can choose to have their items commingled or not. Commingling allows them to possibly offer faster deliveries, and they don't need to add extra barcodes to their products.
Even if comingling is enabled, it seems it's not exactly what HN had made me believe. Amazon still tracks and separately manage each seller's inventory, but might choose to ship you another's seller item if it is closer to you for faster delivery. Again, sellers can opt out of this if they don't want it.
It seems some products aren't allowed to be commingled:
> To qualify, products must: Be in new condition. Have only one scannable barcode that is matched to one ASIN in the Amazon catalog. Have no expiration date. Not be consumable or topical products such as skin creams, shampoos, or cosmetics. Not be dangerous goods.
I think beyond offering sellers the option to opt out of comingling, I'd like to see it offered to buyers as well. At checkout, it could let me know that I have the choice to get it from my chosen vendor's stock which would result in X days delivery time. Or to get it from alternate vendor Y for Z faster days delivery.