Prevent overselling and lost sales with real-time inventory sync between OnBuy and Shopify.
OnBuy stock sync keeps your Shopify inventory and OnBuy listings in agreement at all times. Sell a unit on either platform, and both sides update automatically. No manual stock adjustments. No overselling. No embarrassing cancellations after a customer already paid.
TL;DR: Selling on OnBuy and Shopify simultaneously means your stock levels need to match across both platforms at all times. One missed update and you’re either overselling (angry customer, cancelled order, damaged seller rating) or underselling (lost revenue from listings showing out of stock). Here’s how real-time inventory sync between OnBuy and Shopify actually works.
Scenario: you have 3 units of a product. Two sell on Shopify within an hour. Your OnBuy listing still shows 3 available. A customer buys on OnBuy. Now you’ve sold 5 units of a product you only had 3 of.
On OnBuy, overselling leads to forced cancellations, refunds, and hits to your seller performance metrics. Enough cancellations and your account gets flagged. It’s the fastest way to burn through the trust you’ve built with buyers.
The opposite problem is equally costly, just less visible. If stock updates lag and your OnBuy listings show zero when you actually have units available, you’re leaving money on the table. Those are sales that went to a competitor because your listing looked sold out.
The concept is simple. The execution is what matters.
Your Shopify store holds the master inventory count. When a sale happens on any channel (Shopify direct, OnBuy, or anywhere else), the stock count in Shopify decreases. That updated count then pushes to OnBuy, adjusting the available quantity on your OnBuy listings.
It works in both directions. When an OnBuy order syncs into Shopify, the inventory decreases in Shopify first, then that decrease reflects back on all connected channels. The integration handles this automatically: no manual exports, no spreadsheets, no end-of-day batch updates.
Stock changes come from multiple sources throughout the day:
Every one of these events needs to propagate to OnBuy. An integration that only handles sales but ignores restocks will drift out of sync within days. The best integrations treat Shopify as the single source of truth and push any stock change to OnBuy, regardless of what triggered it.
Stock sync gets more complex when variants are involved. A t-shirt in 4 sizes and 3 colours is 12 separate inventory lines. Each variant has its own stock count in Shopify, and each maps to a separate listing on OnBuy.
If only the “Medium / Black” variant sells out, only that specific variant should show as unavailable on OnBuy. The parent product stays live with the remaining variants. Getting this wrong means either hiding products that still have available variants or showing variants that are actually sold out.
A proper integration maps variant-level inventory, not just product-level totals.
Some sellers keep a buffer between their actual stock and what they list on marketplaces. If you have 20 units, you might list 18 on OnBuy and reserve 2 as a safety margin. This protects against timing gaps: those brief moments between a sale happening and the stock update propagating.
Whether you need a buffer depends on your sale velocity and how fast your sync runs. For most sellers with moderate volume, real-time sync without a buffer works fine. High-velocity sellers moving hundreds of units per day on fast-selling SKUs might want a small buffer to avoid overselling during peak periods.
Here’s the daily reality when sync is working properly:
No spreadsheets. No end-of-day manual reconciliation. No logging into OnBuy’s seller panel to update numbers.
No system is perfect. API rate limits, temporary outages, and connectivity issues can cause sync delays. The question isn’t whether delays will happen, but how the system handles them.
Good integrations queue failed updates and retry them automatically. They flag discrepancies for you to review. They don’t silently fail and leave your OnBuy listings showing stale data.
If you’re evaluating an integration, ask how it handles failures. That tells you more about reliability than any feature list.
Stock sync is one piece of the broader OnBuy Shopify integration. It works alongside product sync (pushing your catalog to OnBuy) and order sync (importing OnBuy orders into Shopify). Together, they let you run OnBuy as a sales channel without running a separate operation.
Read our complete guide to selling on OnBuy from Shopify for the full picture, or head to the Shopify OnBuy integration page to connect your store.