By “vendor”, do you mean the one providing it to the customer? Because Amazon does that to with their FBA service.
The vendors I’m talking about are ones like Frito Lay, Nabisco, Coke, etc. The vendors deliver the product and (sometimes) put it out, but the store knows what’s selling and when. If the vendor wanted that information, they’d have to ask the store (through contracts), or watch the shelves themselves.
And despite what Amazon says, they’re a store. They just blur the line between store and marketplace. The “vendors” in Amazon’s case are the third party sellers. They pay Amazon a percentage of their sale for the ability to sell on Amazon.com. At the same time, Amazon sells stuff they buy and markup themselves (books for example).
In both cases, the product is commingled (white label next to name brand), and the customer doesn’t always know if they’re buying a vendor or a store brand. Amazon will tell you “sold and shipped by X”, but you have to know to look for that.
The vendor is the party you form a contract with. They get liability for product problems. In a store that's the store. On Amazon it might be dropshipper748.
This gives stores an incentive not to stock, say, dangerous electronics from untraceable vendors in China. And it gives you someone to sue if the phone charger burns your house down.
Again. I’m not talking about that definition of vendor. But if you insist to use that one, Amazon can be liable for products sold on their marketplace (at least in California).
For third party sales Amazon insists that they are not the vendor. It's not the same thing.