There is the main Amazon site that everyone is familiar with. You go to Amazon, buy the product from Amazon, pay Amazon, then receive a shipment from Amazon via carrier.
Next is the Amazon seller side. This has two different sides to it. There is the traditional seller model that is akin to eBay's buy-it-now. You go to Amazon, buy from the seller, pay the seller (via Amazon), then receive the shipment from the seller via carrier.
The side to the seller is fulfillment. Sellers have the option to use Amazon as a distributor. It's essentially a hybrid model of the two. You go to Amazon, buy from the seller, pay the seller (via Amazon), then receive the shipment from Amazon (that the seller sent to Amazon) via carrier[1].
My point is that Amazon doesn't exactly put their seal of approval above everything. Just most things.
[1]: https://services.amazon.com/fulfillment-by-amazon/how-it-wor...
If it gets to the point where it's difficult for me to find products of the quality that I want on Amazon, I'm not going to worry about whether it's Sold By Amazon or not - I'm going to find an alternative.
I mean I wouldn't say Amazon is a byword for quality and value, but at the same time I would say most people would put it above, say, ebay and there really doesn't seem to be much of a difference anymore.