Contents7
An online store, a marketplace, a physical shop: three places where a customer can buy the same item, and a single shelf where it sits. As long as sales are rare, manual updates are enough. Once volume rises, the last unit ends up sold twice, and someone has to call a customer to cancel their order.
This guide is for teams that sell on several channels, or are about to, and want accurate stock everywhere without spending their days on it. It is about organisation and rules, not about a particular piece of software.
In short
An oversell costs a disappointed customer, and on marketplaces visibility too: Amazon requires a seller cancellation rate below 2.5%.
Each quantity lives in one place (store, ERP or point of sale), and every channel reads it.
It is available stock that goes out to channels, not on-hand stock.
A safety margin on the last units and an alert on discrepancies catch what the connection lets through.
What an oversell costs
An order you cannot fulfil first costs a disappointed customer. On other people’s channels, it also costs visibility, because platforms measure seller reliability. Amazon turns it into a numbered rule:
A high CR indicates inconsistent inventory management, which can result in a negative customer experience if orders listed as being in stock are later cancelled.
The same page requires a seller cancellation rate below 2.5%, measured over seven days, or seller-fulfilled selling privileges may be suspended. Google, for its part, requires the availability stated in Merchant Center to match the product page. Its automatic item updates can correct a mismatch from the page, but the help warns that they are poorly suited to products whose availability changes more than once a day: the product may then be disapproved rather than corrected.
A single source of truth
The principle fits in one sentence: each quantity lives in one place, and every channel reads it. That place can be the store, the ERP or the point-of-sale system, depending on how you are organised. What does not work is having two.
The path of a sale in synced inventory
- The sale
On the store, the marketplace or at the till.
- The source
It deducts the item, whatever the channel.
- Available stock
Recalculated, reservations included.
- The channels
Each receives the new quantity.
Vocabulary matters as much as the connection. Shopify, for example, distinguishes several inventory states:
Definition
Available inventory
According to Shopify’s help, the inventory you can sell. It differs from on-hand inventory (all units at a location), committed inventory (units set aside that can’t be sold) and unavailable inventory (units set aside by apps or held for other reasons).
It is available stock that must go out to channels, never on-hand stock: a unit already sold online but not yet shipped is still on the shelf, and it is no longer for sale. The full definitions are in Shopify’s help on inventory states.
The shop is a channel like the others
The physical shop is often the link that drifts. A sale at a till not connected to the same stock, an item put aside for a customer, a damaged unit taken off the shelf: so many gaps the online store cannot see. Three habits limit them:
- A till connected to the source, deducting at the moment of sale rather than at the end of the day.
- One location per site (shop, warehouse, stock held by a provider), to know where each unit is and where the order will ship from.
- Cycle counting: a few references recounted each week, rather than one big annual stocktake that finds discrepancies too late.
Real time or batches
Not all connections run at the same speed. The choice depends on volume and channels.
Update on every sale
The gap between channels lasts a few seconds
Requires connectors each platform accepts
A connection failure shows up quickly
Batch updates, hourly or nightly
Simpler to set up
A product sold in the morning can stay on sale elsewhere until the next batch
Suits large, low-pressure catalogues
In both cases, keep a safety margin on the last units: showing zero on a marketplace when one or two remain avoids most double sales, at the price of a few lost sales. The threshold is set product by product, according to how fast it sells.
What breaks a sync
Watch out
A silent sync is worse than a broken one
A stopped connection gets noticed. A connection that keeps running while sending wrong quantities goes unnoticed until a customer complains. An alert on discrepancies is worth more than blind trust in a job that shows green.
The most frequent causes:
- References that do not match. A variant (size, colour) must carry the same code on every channel; otherwise the sale of one model deducts the stock of another.
- Returns not put back. An item that came back and passed inspection must return to available stock, or leave it if it cannot be sold. Our article on product returns details that circuit.
- Quantities edited by hand on one channel, which the source overwrites at the next batch, or the reverse.
- An empty response taken for zero stock, which pulls an entire catalogue off sale.
From the field
On Palmora Property, the real estate portal we run, the property catalogue flows from the CRM to the website. A rule there refuses to publish an empty catalogue: without it, an empty API response would be enough to put a site online with no properties at all, with a job reported as successful. The same caution applies to stock: a quantity that drops to zero across the whole catalogue at once is a failure to check, not a stockout to publish.
Where to start
- Choose the source.
The store, the ERP or the till: one place is authoritative for quantities.
- Unify references.
One code per variant, identical on every channel.
- Connect channels one at a time
Checking quantities after each addition.
- Set the safety margin
On fast-selling products.
- Monitor discrepancies
An alert when a channel’s quantity diverges from the source, and regular recounts.
Stock is only part of the data that flows to channels: prices, identifiers and availability make up the feed described in our guide to preparing your store for Google Shopping and marketplaces. Wrong availability gets products disapproved there, which also makes a Google Shopping campaign sink. If you open Amazon, the question of shared or handed-over stock comes up from day one: see selling on Amazon alongside your store. The sync tool, finally, is often a paid app: our method for weighing Shopify apps applies, and the choice between Shopify or Medusa decides what is built in and what has to be developed.
Accurate stock then shows everywhere: on the product page that converts, which states a delivery time you can keep, in the five numbers to track every week, where a stockout pulls conversion down, and in abandoned cart reminders, which must never lead back to a sold-out product. The same rules apply abroad, when the store becomes multilingual and multi-currency, and asking for a review after delivery assumes an order that actually shipped, as our article on customer reviews points out.
If your channels contradict each other on quantities, our ecommerce service starts by mapping your stock sources before connecting anything.
Frequently asked questions
How do you avoid selling the same item twice on several channels?
By keeping a single source of truth for quantities, sending channels available stock rather than on-hand stock, and holding back a safety margin on the last units of fast-selling products. Updating on every sale narrows the gap; an alert on discrepancies catches what the connection lets through.
Do you need an ERP to sync inventory?
Not necessarily. An online store acting as the source, connected to the till and the marketplaces, is enough for many businesses. An ERP becomes useful when purchasing, several warehouses, production or accounting need to share the same quantities. The criterion is not the size of the tool, but having one place that is authoritative.
How often should you update stock on marketplaces?
As often as possible for fast-selling products or those with few units left, ideally on every sale. For a large, low-pressure catalogue, regular batch updates can be enough, with a safety margin. Check what each platform accepts in terms of frequency and format.
What happens to stock when a customer returns an item?
Inspect it before putting it back on sale. An intact item goes back into available stock, and the source sends the new quantity to every channel. A damaged or incomplete item leaves available stock: it is repaired, sold another way or written off, but never listed as new.
Sources
- Google Merchant Center Help, automatic item updates (new tab)support.google.com
- Shopify Help, inventory states (new tab)help.shopify.com
Jim · founder of Brain Plus AI
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