On a listing with several sellers, the shopper sees one offer up front: the Buy Box. Most sales go to whoever holds it. When sellers offer the same price with good standing, Amazon rotates the Buy Box among them, so each one wins part of the day. Your share of that rotation is what you can sell.
Who can win it
Not every offer on a listing competes for the Buy Box. Count the sellers who really share it:
- sellers shipping through Amazon's warehouses at the Buy Box price count fully;
- sellers priced above it count for less, since they win it only now and then;
- sellers who ship their own orders count for about half at the price, and not at all above it;
- any seller who won the Buy Box in the past month counts fully.
Amazon is different: when Amazon sells on a listing it tends to keep the Buy Box, and its share comes off the top before the rest is split. See When Amazon sells on the listing.
Your share
Your share is what is left after Amazon, split between you and the sellers who can win it, and only for the hours the listing has a Buy Box at all. Count the sellers you expect when your order arrives, not only today's: if the offers box shows sellers joining, your share is smaller by then. Run the PO Assistant on the product and a pin on the offers box says so, such as "+5 sellers".
You won't always win your share. A seller who drops the price takes more of the rotation for a while. That's why it pays to size on a middle figure, not your best week, and to keep a first order small.
The PO Assistant's suggested quantity uses the simple version: the listing's monthly sales split evenly between you and the sellers on it, for the days an order should last. It doesn't weigh who can really win, so check it against the offers box. See what a read gives you.
Crowded listings
Crowded listings mean small shares and a higher chance of a price war. Many buyers keep a ceiling on sellers, and the Results rules have one: FBA sellers hides listings with more Buy Box eligible sellers than your number. See What the rules test.
Don't set it too tight. A crowded listing that sells a lot can still be worth a small order, and your profit and ROI minimums often do the sorting for you.
A worked example
A listing sells about 600 a month and has a Buy Box 95% of the time. Amazon holds it 20% of the time. Four other sellers can win it. Your share is what Amazon leaves, 80%, times 95%, split five ways: about 15%, or 90 a month for you.
If two more sellers join before your order arrives, the split is seven ways: about 11%, 65 a month. Size the order on that. For 45 days, that's about 98 units, so with cases of 12 you'd buy 8 cases (96), not the 11 that today's count of sellers would suggest.