AFAIK, the distinction is that instead of the seller having control of the price the item is offered for, AZ gets to set the price, subject to a minimum price, selected by AZ, that's guaranteed to the seller, even if AZ discounts the item below that price. I've once in a while bought product where there's some note to the effect that AZ is kicking in some contribution to lowering the price, which seems to correspond with this SBA program.
There's some interesting detail on the SBA program here: https://www.ecomcrew.com/the-a...
Prices on AZ change all the time, as you can see from camelcamelcamel.com, or by putting products in the "save for later" list and observing the updates on prices when you're about to checkout. It was just as controversial when AZ switched to the "agency" model for e-books under pressure from Apple, where the publisher got to choose pricing instead of AZ - but books are a different kind of product, as there isn't really a brand-X replacement for a particular book by a particular author, unlike laundry soap or a USB cable - but there are multiple formats e-book, paperback, hardback, library binding, etc., and for some formats, it seems AZ gets to choose the sales price, and others, the sales price is set by the publisher. Many AZ prices are changed dynamically by algorithms that sometimes go to strange places, as when seller X sets the price at 105% of seller Y, and seller Y sets the price at 120% of seller X, and ordinary products start getting priced at extraordinary prices. https://fortune.com/2020/07/14...