Widget usually sells for $25. Amazon agrees to buy 1 from you at $20 (you give them a volume discount because it's Amazon buying so you figure it'll sell like hotcakes. You still make a profit at $20.). Amazon then prices your widget on their marketplace for $30. Seems fine so far. The trick is that Amazon turns around and sells their widget, made by an alternate supplier, for $23.
Your deal with Amazon means you no longer have control over how much your widget sells for, just that you're selling them to Amazon at $20 and Amazon gets to charge however much they want.
To no one's surprise, your widget, priced at $30, doesn't sell because Amazon's is $23. You're not allowed to turn around and sell them on Amazon's marketplace at all, so they just sit there at $30 and don't sell.