They did bought the goods from him beforehand. And he agreed to sell. He should have considered the possibility that Amazon has option to later sell the items as a higher price. Isn't that obvious to any seller?

Amazon does not buy and keep a stock, they charge you for the privilege of selling in their marketplace:

> Sellers then bore the risk of having their products not sell in a timely manner, or at all, while still paying Amazon for things like storage fees of their enrolled products