They certainly have stopped people doing it at prices cheaper than they sell through Amazon. They're running a protection racket.

They don't need to. They have such a big chunk of the market that if you don't sell how THEY want you to, you lose access to a huge chunk of the market. That's the problem. That's the complaint. That sellers who want to sell to buyers who want to buy are forced to do it how Amazon states, for the benefit of Amazon, which means the seller has to put their prices up to cover Amazon's Danegeld, but then may NOT also sell elsewhere at the lower, non-Danegeld price.