What stores are you going to that have third party sellers? A better analogy is a mall.
Nearly every grocery store in the US has third party sellers. They pay the store for shelf space, provide their own stockers, etc. It's not as different as you'd believe, because you've 'heard' of the brand names doing it.
A good example that I see at my local grocery store all the time is Coca-Cola.
Yes, especially common in the bread and chip aisles. Third-party stockers come in and replace the inventory. Same with soda.