Sync real-time inventory levels across Amazon, eBay, and a Shopify storefront using a centralized inventory ledger

Steps

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.