If the vendor for the $9 offers it for $8.50 elsewhere and Amazon starts focusing on a $9.50 offer is that "better for the consumer"?
If the $9 doesn't exist and there are two vendors at the $9.50 price and one has a higher review history but offers it lower elsewhere is Amazon making things better by focusing on the other one?
If another site starts selling the same product for $8.50 (regardless of who the seller is), Amazon may hide the buy box and put all the offers in the "other offers" section to avoid advertising an offer that isn't the best price available online.
Consumers price compare across sites, and Amazon wants to maintain its reputation for low prices (and in truly egregious cases prevent sellers on Amazon from price gouging).
If I don't see a price I assume it is out or they only have used ones. Certainly occasionally I have seen scalpers when I double checked anyway to see what the price would be if it returned but generally Amazon caught maybe half the scalpers in my experience.
Having a signal that means "you could get it less elsewhere" be the same signal as "I only have scalpers" isn't exactly good for consumers.
Also this implies that Prime shipping is worthless given they exclude it from the calculation...
Is buying from the buy box ever to the customers advantage?