The core problem is that the market owner and operator is also a market competitor. It's just not believable that they wouldn't cheat.

They have sales data from all their competitors on this market- numbers, prices, trends, across many different competing companies in many different sectors. They have priority access to consumer eyes by owning the home page, owning the search results, and owning the customer relationship.

And all of this wouldn't be that big of a problem if Amazon wasn't the absolutely dominant player in the still-growing e-commerce market. Because at that scale, these problems become societal problems that governments have to address.