The 'Add to Cart' box on an Amazon listing. When multiple sellers offer the same product, the Buy Box winner takes the sale by default, so holding it is the difference between owning your listing and watching someone else monetise it. Winning is driven by price, fulfilment method and seller performance, and FBA carries a strong structural advantage: in most categories the Prime badge beats a lower price from a non-Prime seller, because customers filter by Prime. The Buy Box is also where knockoff pain lands - a hijacker undercutting you takes sales on the page you built, which is why Brand Registry and counterfeit reporting matter.
Benchmark. The qualitative test: you hold the Buy Box on your own branded listings. If you are losing it, check fulfilment method first - FBA almost always wins for discoverability and the Buy Box - then find out who the competing seller actually is.