If Amazon wasn't directly competing against their sellers, and doing exactly what you said, while lying to Congress about it [1], then maybe journalists wouldn't be writing about Amazon stealing secrets from sellers.
Clearly you have to be a megacorp to get it both ways. Amazon shouldn't be able to evade liability for products sold on their website if they compete against their "sellers" as well.
1. https://www.wsj.com/articles/amazon-scooped-up-data-from-its...