When you divert inventory from FBA to FBM, SP‑API’s job is to keep all the stock numbers in sync fast enough that you never “sell” the same unit twice.
In practice, your app (or tool) listens for new orders and inventory changes, then immediately updates the relevant offers through SP‑API so FBA and FBM never show the same sellable units at the same time. For example, when an FBM order comes in, your system subtracts that quantity from your FBM stock and, if you’re drawing from the same physical pool, also reduces any linked FBA‑reserved or “available” quantity in your own database, then pushes fresh inventory numbers back to Amazon.
The key is near real‑time syncing: SP‑API lets you continuously read and write inventory levels across fulfillment types so your listings reflect true available stock instead of lagged, conflicting counts.
