I have no official source, only experience. My company has been selling on Amazon for over 6 years (we're medium-ish volume, 200+ orders a day on Amazon alone).
We're found majority of customers have no idea who they're buying from. Majority assume it's always Amazon, and often contact us about other orders they've recently placed, etc.
If you browse around Amazon, you'll notice items that have many "offers", clicking this link will show you the seller's who are listing that product. You'll notice how few Amazon.com is actually listed as a seller. Typically they're listing the top 10 items in any given category, but there's millions of products on Amazon, and very few they actually carry. When they are listed on an item, they're not always the best deal anymore.
We don't worry about competing with Amazon.com anymore, we worry about competing with our Chinese manufacturers (sometimes the same ones we source from!), who get USPS/US Gov't subsidized shipping (free shipping from China, and product costs ~1/2 of a US seller).
Amazon is transitioning into a seller platform, not a seller themselves. All the warehouses they keep popping up everywhere are mostly for FBA (Fulfilled by Amazon) warehouses, where 3rd party sellers send their products so that Amazon can warehouse them and ship them when there's a sale.
Amazon FBA is a fairly competitive fulfillment cost, sometimes beating our own internal fulfillment costs (we're stepping up our FBA game because of this, but unfortunately will ultimately result in fewer warehouse staff in our company).
Another secret - Amazon is rarely the best deal for any given item. Officially, policy says you must offer the "best deal" on Amazon, but this is never enforced in our experience. Amazon takes a commission of up-to 30% (depending on the item category), so unless you already have very healthy margins, you end up baking in an additional 30% to the retail price. Customer's pay your commission fees. So, if you find a product you want, see who the seller is, and try to find their website outside of Amazon.com. Chances are, the item will be up-to 30% cheaper (sometimes more!). Of course, this isn't an always guarantee, but it pays to check.