You might reasonably expect that Amazon only allows one Buy Box winner at a time, however Amazon increasingly allows for multiple Buy Box winners at one time on any given ASIN.
Amazon will sometimes distinguish Buy Box winners by condition, allowing for both a new and used condition winner.
More frequently, Amazon will split or share the Buy Box based on Prime status, allowing for one winner for Prime customers and another for non-Prime customers. A non-Prime offer will never be eligible for the Prime Buy Box, but a Prime offer is often both the prime and non-prime winner.
Every share of the Buy Box matters! With Market Labs Alerts, you'll have visibility to exactly what is happening with the Buy Box. You'll know if you are winning or losing the Buy Box entirely, sharing both Prime and non-prime between your own offers, or sharing the Buy Box with a competitor.