Amazon deliberately obscures that. They can't have it both ways; they present these sellers as part of their store, so they should take responsibility for the goods they sell.

There is a tiny 'sold by X and shipped by Y' somewhere on the right in small print. I'm not sure that that's sufficient, but it is there.

What is really lacking is a filter to get rid of all results not sold by amazon.

Unfortunately, even buying from Amazon (with eg Amazon.co.uk as seller) doesn't help with this completely, because of Amazon's stock co-mingling setup.

Even buying from Amazon itself is no guarantee because they commingle inventory. The OP should totally escalate to Amazon customer service for a full refund on a fraudulent product, but you get the feeling Amazon is fully aware of the situation and profiting from it, and treats the occasional refund as a cost of doing business.

Do you have a source for this?

The inventory mingling is a well known problem. Just search it up. The fact that Amazon won’t fix it and does it on purpose leads to a relatively narrow range of conclusions.