If inventory was commingled, and you might not even get the item from the seller you picked, what was the point of the 5-star rating system?
Was it meant to rate the product, not the seller? If so, that’s probably not how most people understand it.
Yes, in Amazon's world, the star rating is for the product. Which is especially confusing on product pages with vastly different variants.
I always took the product rating as rating the particular item (regardless of where it was manufactured) and the seller rating as rating interactions regarding the sales/support/return process. I'm not sure what good having both ratings be the seller rating would be, or why one would look at the ratings of a single product to judge a seller's rep and vice versa.
The main problem I have with the way Amazon product ratings are structured is the grouping of products under a single rating. Particularly with electronics, e.g. the 32" variant of a monitor might as well be a completely different product from another manufacturer when compared to a 27" variant from the same product family - yet there can be a dozen variants under a single rating.