Difference is that in retailers, they take on the risk and product research. They buy the items from manufacturers, sell it and then sometimes they make their own cheap unlabeled versions (usually buying from the same manufacturer).

Amazon is offering a big open marketplace where they invite people to offer goods, then they observe which goods sell well and how are they made, then the push out the sellers and they offer their own versions of it, essentially pushing the risk onto sellers and only reaping the benefits.

They are abusing the trust of sellers on their platform and they get away with it because of their sheer size.