Though now we know to the degree that it actually happens. So here we go. If you are to search for something along the lines of 'mens button down shirts' or 'paper towels' a large amount of the products that will be shown to you are products that are owned by Amazon through a private label brand. That's not something that is shown to you, as they are displayed alongside the likes of other brands.
All of this is happening while the EU is currently looking into this very issue, and they're seeing if there is anything wrong with a company owning a marketplace and selling products on that very marketplace . I'm assuming they're looking at this under an antitrust lens, as Amazon also controls about 37 percent of all online sales.
Here is the WSJ article as well - https://www.wsj.com/articles/amazon-changed-search-algorithm-in-ways-that-boost-its-own-products-11568645345?mod=hp_lead_pos5
For me I don't see anything wrong in this except there is a superior argument to the contrary. Companies can acquire another company for several reasons. I have read how a company acquired another company that provides its raw material and this is due to the fact they want to ensure consistency in supply of the raw material. Companies have always been biased against companies that is not related to them compared to one in which there is an established relationship just like human endeavors. So, if Amazon can have a company that produces what the client is looking for and they suggested in addition to other sellers, so far they are not forcing customers to buy from their preferred supplier, then there is nothing wrong in it.