After a certain size, they should be regulated as public utilities. So many companies completely depend on Amazon's platform for their business and they're all easy prey for the behemoth.
I did some work for a company that sells batteries on Amazon. A simple dispute got them suspended across the board. A decision made by a third rate employee from god knows where, probably without even a cursory reading of the case, made in a split second, brought down a company with 10 years in the market and hundreds of employees.
No due process, no rights, nothing. You can't defend yourself, they just shut you down and then you have to beg for weeks to be let back in. After weeks and hundreds of thousands in losses, you have no legal recourse against a wrongful suspension.
You can make the argument that they could just set up their own online store for the batteries and you're right, they can. But amazon and ebay are so big that its practically impossible to sell these things at scale without them. Its not a fair game and this is but one of the issues.
They definitely need to be treated as public utilities after a certain amount of users.