Amazon is a retailer. The fact that some of their goods are never physically in their hands doesn’t stop them from being the seller.

Amazon bears responsibility for what happens on its property, particularly when it has been notified thousands of times over of shady goings-on.

If it is on its platform, it takes payment, and often provides fulfillment, and shipping. It does indeed sell the items that third parties put on it. See the injury suit from the dog leash, where Amazon was indeed liable.