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.
The “Sold by Amazon” program resulted in prices for some products increasing when Amazon programmed its pricing algorithm to match the prices that certain external retailers offer to online consumers.
As a result, when prices increased, some sellers experienced a marked decline in the sales and resulting profits from products enrolled in the program. Faced with price increases, online customers sometimes opted to buy Amazon’s own branded products — particularly its private label products. This resulted in Amazon maximizing its own profits regardless of whether consumers paid a higher price for sales of products enrolled in the “Sold by Amazon” program or settled for buying the same or similar product offered through Amazon.
But regardless, even if they are allowed to leave the program, that doesn’t make it any less illegal.
A wholesaler sells to a reseller at their usual price. Reseller deliberately sells less than RRP & sometimes less than their buy price. It's a good way to guarantee the reseller "owns" the market. Then, if the reseller is big enough and has captured the market, has the leverage to tell the wholesaler to lower the wholesale price and/or the reseller gets it made elsewhere and becomes the wholesaler.
It's pretty nasty.
It happened to my family business. A big box chain sold the same products for less than their buy price. We were close friends with the wholesaler ( a timber mill), and the sales rep even went to the big box to get them to lift their price.
2 years after shutting us down, the big box store sells the same product, from the same source, for 50% more than we ever used to sell it for.