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Spread, landed, and whether it holds
Any tool can show two different prices. The questions that decide whether a unit is worth buying are whether the gap survives shipping and whether it has ever held for longer than a day.
Resale & arbitrage
Landed or nothing
Spread is computed after shipping, because most apparent arbitrage is a shipping-cost illusion.
Resale & arbitrage
Durability
History shows how long a gap has persisted before. A spread that has appeared and closed within a day eleven times is not an opportunity.
Endpoints that do this
Where it lands in the API.
Compare
One product across every seller that carries it, ranked by landed cost, with the spread. Server-side work is worth more than four raw reads the caller has to join.
Watches
A standing condition on a product, a seller, a brand or a whole category.
History
The observation series. Append-only — nothing is ever updated, so a replay reconstructs exactly what you were told and when.
Questions
Common questions.
Can I be alerted on a spread rather than a price?
Yes. A watch condition can be expressed on the cross-seller spread, so it fires when the gap opens rather than when any single price moves.