https://www.amazon.com/gp/offer-listing/B000EYP57K/ref=dp_ol...
Buy from Amazon - the second choice. Pretty sure that is the default purchase option on the main page as well.
Just because you buy someing from Seller A, even if Seller A is Amazon LLC, you may not be getting stock from Seller A
If I am A retailer selling SuperWidget 55, and I ship Amazon 100 Genuine SuperWidget's to store in their warehouse for amazon to ship to my customers (aka Amazon Fulfillment Service) . Then another competitor ship 100 Fake SuperWidgets to amazons warehouse. All Amazon records is they have 200 SuperWidgets.
When a Customer orders a SuperWidget, they will get what ever SuperWidget is closest to their location, and a random one from that location, it may not be my SuperWidget but one of the 100 fake ones, even though the customer selected to order from me.
So then the next question is why would a retailer use Amazon Fulfillment, well if you want to get any Prime buyers you must, as if you do not use Amazon Fulfillment your orders are not "Prime Eligible" which eliminates a huge number of consumers.
You implied that sellers are required to use it.
Edit: it's a different user that replied to me, my mistake.
Also, Amazon tracks which order came from whose inventory, so if it's reported as fake they can punish the right seller. Any system of that size is going to have some slip through, but Amazon does a lot to vet sellers for specific products, categories, and so on. Just recently certain iPads were restricted, and I couldn't get approved with retail receipts, they wanted invoices from authorized distributors, for example.
As a buyer you can't do much about comingling, but that shouldn't be relevant except for products with an abnormally high counterfeit rate. And you will get your money back with very little hassle when buying FBA (they refund you immediately and then require you to return it), and it helps hurt the bad seller. The fact is the vast majority of purchases on Amazon are legitimate. I agree Amazon needs to do something about their image, but it doesn't seem to be hurting their sales. And who else are you going to go to? eBay is even worse, and retailers are more expensive most if the time.
If you order from Seller A on "Fulfilled by Amazon", then Amazon will ship you from the nearest warehouse that has the same product, even if it's technically stock of Seller B.
And Seller B might have handed a crate of counterfeits to Amazon. Angering both the customer and Seller A who has no way to actually enforce the quality of what they sell on Amazon.
Pros to comingling:
1. Makes customers happier to get their item sooner
2. You don't need to label your items, which either saves you time or saves you the 20 cents per item fee Amazon charges
3. Your items get processed quicker. If your items are in between warehouses, they can still be "shipped" to the customer.
Cons:
1. Counterfeit possibility. This varies by item. Amazon claims to track which seller inventory was sold from.
2. Bad returns, may hurt your account or require time to deal with.
Basically sometimes it makes sense and sometimes it doesn't. I use a prep center which labels for me, and when I send in items myself I have Amazon label them.