A big complaint I've seen in the past is AmazonBasics, where amazon copies successful products and sells them for slightly cheaper. This is identical to brands like Great Value (walmart), Up&Up (target), Nice (walgreens) and Safeway Select.
Similarly, I have seen a lot of complaints that amazon charges for ad space in the search, and you therefore need to pay to get out of page two. This, again, is widely practiced in retail. The cereal aisle doesn't look like it does because the store manager is making it up, general mills and post rent the space and decide how to arrange their sections. New retail products need to explicitly or implicitly pay for placement: either with cash or with free / discounted product.