The Buy Box — now officially called the Featured Offer — is the box on a product page containing the Add to Cart button, and it determines which seller receives the sale when several sellers list the same ASIN. Amazon awards it on price, fulfilment method, delivery speed, stock position and seller performance. It matters to advertisers because Sponsored Products generally will not serve for an ASIN where you do not hold the Featured Offer.
How it is awarded
Amazon does not publish the weighting, but the inputs are well established: landed price including delivery, fulfilment method (FBA and Prime-eligible offers are advantaged), delivery speed, stock availability, and seller performance measures such as order defect rate.
On a listing with a single seller the Featured Offer is normally yours by default. It can still be suppressed — usually when Amazon judges the price uncompetitive against the same product sold elsewhere.
Why it decides whether your ads run
Losing the Featured Offer typically stops Sponsored Products serving on that ASIN while the campaign continues to look perfectly healthy in the console. Impressions fall away with no error and no obvious cause.
It also suppresses conversion sharply when ads do serve, which surfaces later as an unexplained ACOS spike. Checking Buy Box percentage is the cheapest first move in almost any sudden-performance investigation.
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