The barrier to entry in that market (Amazon and its competitors) is staggering compared to the barrier to entry into the general "sell stuff online" market.

I also don't agree with your list of competitors. Amazon is more like a mall, while almost uniformly the others you list are more like a retailer acting as a middle-man between wholesalers and the general public. I'd agree with Aliexpress and maybe Walmart.

It's just not a healthy market.