How to Automate Inventory Sync Across Multiple Channels

Selling on multiple channels is how you grow. But managing inventory across Shopify, Amazon, eBay, your wholesale portal, and your physical warehouse using spreadsheets is how you drown. Overselling costs the average multi-channel seller $4,200 per month in cancelled orders, refund fees, and damaged seller ratings. The solution is automated, real-time inventory synchronization that treats all your channels as a single unified stock pool.

This guide walks through the architecture and step-by-step setup for a multi-channel inventory sync system using Make.com as the orchestration layer.

Step 1: Establish a Single Source of Truth

The most critical decision in multi-channel inventory is choosing your single source of truth (SSOT). Every inventory update across every channel must flow through this central system. Your options:

• Your ERP/accounting system: QuickBooks, NetSuite, or Cin7 if you already track inventory there.
• A dedicated inventory management platform: SkuVault, Finale Inventory, or Linnworks if you need advanced warehouse features.
• Your primary e-commerce platform: Shopify can serve as SSOT for smaller operations, but this becomes limiting as you scale.
• A custom database: Airtable or a PostgreSQL database if you need maximum flexibility for custom logic.

Never use a marketplace (like Amazon) as your source of truth. Marketplace inventory APIs have rate limits and delays that make them unreliable as central systems. Always push to marketplaces, never pull from them as primary data.

Figure 1: Hub-and-spoke model where orders flow into the SSOT and stock updates push outward

Step 2: Configure Event-Based Order Ingestion

Every sale on every channel must immediately decrement your central inventory. Set up webhook listeners for each sales channel:

• Shopify: Use the orders/create webhook. This fires within seconds of a purchase, including the line items, quantities, and variant IDs.
• Amazon: Use Amazon SP-API's order notification or poll the GetOrders endpoint every 5 minutes (webhooks are not available for all regions).
• eBay: Use eBay's platform notifications for the FixedPriceTransaction event type.
• Manual/wholesale orders: Trigger via form submission, email parsing, or PO processing (see our PO automation guide).

Each incoming order should decrement the available stock in your SSOT immediately. Use atomic operations if your data store supports them to prevent race conditions during high-volume periods like Black Friday.

Step 3: Build the Outbound Stock Push

When your SSOT inventory changes, you need to push the updated quantity to every connected channel. This is where most DIY implementations fail. Here is the correct approach:

• Calculate available-to-sell per channel: Total stock minus safety buffer minus channel-specific allocations. For example, you might reserve 20% of stock for Amazon to avoid FBA stockout penalties.
• Batch updates efficiently: Amazon and eBay have API rate limits. Instead of updating one SKU at a time, batch your updates. Amazon's Feeds API accepts XML feeds of up to 10,000 SKU updates in a single call.
• Handle SKU mapping: Your internal SKU might differ from your Amazon ASIN, Shopify variant ID, or eBay item number. Maintain a mapping table that translates between them.
• Set sync frequency: Webhook-triggered (real-time) for Shopify, every 5-15 minutes for Amazon and eBay due to API limitations.

Step 4: Implement Safety Buffers and Low-Stock Alerts

Automated sync eliminates most overselling, but network delays and API lag mean you need safety margins:

• Global safety buffer: Hold back 5-10% of available stock from all channels. This absorbs simultaneous sales during sync delays.
• Channel-specific buffers: Increase the buffer on channels with slower API sync (e.g., 15% on Amazon if you can only poll every 15 minutes).
• Low-stock alerts: Configure automated notifications when any SKU drops below its reorder point. Route these to Slack, email, or your procurement team's dashboard.
• Auto-delist threshold: Set a minimum quantity per channel below which the listing is automatically set to zero or paused. This prevents overselling of your last few units.

One of our e-commerce clients reduced their oversell rate from 8% to 0.2% simply by adding a 10% safety buffer and switching from hourly to 5-minute sync intervals on Amazon.

Step 5: Build Reconciliation and Monitoring

Even the best automation needs verification. Build a daily reconciliation workflow that:

• Pulls current stock levels from every channel and your SSOT.
• Compares expected quantities (SSOT minus buffers) against actual listed quantities on each channel.
• Flags any discrepancy greater than your tolerance threshold (e.g., 2 units).
• Auto-corrects minor drift by re-pushing the correct stock level from your SSOT.
• Generates a weekly inventory accuracy report for your operations team.

For monitoring, use Make.com's scenario execution logs combined with a Slack notification for any failed sync. Track your key metrics: sync latency (time from sale to channel update), oversell rate, and stockout rate per channel.

What to Expect After Implementation

Businesses that implement this architecture typically see:

• 95-99% reduction in overselling across all channels.
• Inventory accuracy above 99% compared to 85-90% with manual tracking.
• 10-20 hours saved per week previously spent on manual stock updates and spreadsheet reconciliation.
• Improved seller ratings on Amazon and eBay from fewer cancellations.

Use our cost calculator to estimate your overselling costs, or explore how automated shipping label generation can further streamline your multi-channel fulfillment.