Perhaps that is already happening, with a select few manufacturers. But consider that many marketplace sellers are third parties (e.g. an electronics shop). Or other manufacturers with different costs (many factors affect this... ___location, workforce..). What happens with these?

I do wonder if Amazon are making any kind of profit on these items. With their long term strategy they can very well afford to make a loss just to undercut the competition and put their products out there, make a name.

They can do cost/benefit analysis. They don't have to pay themselves 15% of the price including shipping in fees, they have good relationships with cheap overseas manufacturers (in China you pay literally pennies for plastic stuff in large quantities) utilizing economy of scales, they know the exact demand/sales of their competitor/"partner", probability they can convert competitor's sales into their own sales, control the visibility on their platform etc. I am sure they make larger profit than what they would get by taking a cut from a 3rd party seller. Or they are irrational, which I don't believe.