The "buy box" is a product-level feature, not a seller-level feature. On any given product page, there may be multiple sellers with offers (one for $9, two for $9.50). The $9 offer would be prominently displayed in the buy box and the other two would be in the "other offers" section.
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).