Establish a central inventory ledger (a database table or cache) with per-SKU available quantity, a safety-stock buffer, and per-channel allocation records — the ledger is the system of record, not any individual channel.
Subscribe to channel order webhooks: Shopify orders/create, Amazon SP-API Notifications (ORDER_STATUS_CHANGE via SQS), and eBay Marketplace Account Deletion/Order notifications — each webhook triggers a ledger deduction.
On each order event, atomically decrement the central ledger quantity for the SKU, then fan out channel-specific inventory update calls: Shopify's inventoryAdjustQuantities GraphQL mutation, Amazon SP-API patchListingsItem with fulfillment_availability, and eBay Sell Inventory API updateOffer.
Implement an oversell guard: before confirming an order on any channel, check ledger availability; if quantity is zero, push a zero-inventory update to all channels immediately to close the listing.
Run a scheduled reconciliation job (hourly or daily) that reads actual warehouse quantities from your WMS/ERP and compares them to the ledger — correct drift caused by returns, adjustments, or missed webhooks.
Handle webhook delivery failures with a dead-letter queue and replay mechanism — failed inventory deductions must be retried with idempotency keys to avoid double-deductions.
Known gotchas
Amazon SP-API inventory updates via patchListingsItem are eventually consistent and can take several minutes to surface as updated offers — do not rely on immediate read-back to confirm the update.
eBay's Sell Inventory API requires inventory to be managed at the inventory item level (SKU) and the offer level separately — updating inventory at the item level does not automatically update offer availability; call updateOffer after updating inventory.
Safety-stock buffers must be tuned per channel velocity — a buffer too small causes overselling on high-velocity channels; too large causes unnecessary stockouts; monitor sell-through rates per channel and adjust buffers dynamically.
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