One possible line is that you can't have a vendor-vendee relationship between two parties if one of them doesn't know the other exists. It's nonsensical to claim that Amazon is facilitating a relationship between manufacturer and buyer if we take at face value the claim that they don't even know who the manufacturer is. At that point, Amazon effectively owns the product and is a reseller.

Let’s keep it simple. Whoever takes the money is the vendor. Amazon lists it, amazon fulfills it, and they get paid for it. Sounds like the vendor to me.

Except Amazon doesn’t list it. The vendor does.

I exclusively use Amazon and fulfilled by Amazon under the assumption they have some sort of QC in place since it's their warehouse, payment processor, and customer support that I have to deal with it. But no, doing that does not guarantee me that I won't be sent questionable goods.

Ebay on the other hand I am under no illusion that they are not the vendor. Same with Craigslist, Etsy, Tindie, etc.

...except the listings that are "Fulfilled by Amazon"...no?

Amazon doesn't add the items to their inventory. Someone who has one to sell (a VENDOR) lists the item for sale with Amazon. If it is "Fulfilled by Amazon", that just means that the vendor sent the item to one of Amazon's warehouse, and then Amazon send the item to the buyer after the sale.

If it isn't "Fulfilled by Amazon", when the item is sold, the vendor ships the item directly to the customer.

If instead of the seller's name the only thing eBay told you was "sold on eBay market", I bet they would be liable too.