How four numbers for one SKU happen

Nobody decides to keep four different stock figures. It happens because each marketplace stores its own quantity, and each one only learns about its own sales.

Sell a unit on eBay and eBay knows. Amazon does not. Shopify does not. Unless something tells them, they continue advertising a unit that is now on a van. Manual reconciliation — someone adjusting the numbers each morning — closes the gap once a day, which means the gap is open for the other twenty-three hours.

The drift compounds in a specific way that makes it hard to spot: it is always in the direction of overstating availability. Every channel except the one that made the sale is now too high. That is why the symptom sellers notice is overselling rather than lost sales.

The single source of truth

The fix is structural rather than clever. One system holds the real number; every channel is a consumer of it, not an owner of it.

That means:

  • Items, packages, suppliers and price lists live in one place, not in whichever marketplace you happened to set up first.
  • A sale on any channel decrements that one count.
  • Replenishment — a purchase order received into stock — increases it, and every channel benefits at once.
  • Nobody edits stock levels directly in a marketplace, because that immediately creates a second version of the truth.

That last rule is the one that gets broken. If a member of staff can adjust a quantity in Seller Central to fix something urgently, they will, and the system is quietly back to two numbers. The discipline is as important as the software.

The timing gap nobody mentions

Even with one central count, there is a window between a sale and every channel reflecting it. It is short, but it is not zero, and on fast-moving SKUs it is where overselling lives.

The practical mitigations are unglamorous:

  • Decrement on order pull, not on dispatch. The unit is committed the moment it sells, not when it physically leaves. Waiting until dispatch leaves the count overstated for hours.
  • Hold a buffer on volatile SKUs. If a product can sell ten units in an hour across three channels, listing the last two is optimistic. Keep a small reserve out of the advertised figure.
  • Watch the fast movers specifically. The risk is concentrated in a small number of SKUs; treating the whole catalogue with the same caution just costs you sales on the slow ones.

There is more on this in the overselling guide.

Purchase orders are part of inventory, not accounting

Replenishment is often treated as a finance activity that happens elsewhere, which is why stock figures go stale after a delivery arrives. It belongs in the same system as the stock.

A workable loop looks like: raise a purchase order against a supplier; the goods arrive; receive them into stock; the central count rises and every channel can sell them. If receiving is a separate manual step in a separate tool, there is a period where you own stock you are not selling — the opposite failure to overselling, and much less visible.

Keeping supplier ledgers and dues alongside means the question "what do we owe this supplier and what did they actually deliver" has one answer.

Bundles, packages and the decomposition problem

The moment you sell a multi-pack or a bundle, one sale consumes several underlying SKUs. If the system tracks the bundle as its own item with its own count, the components silently go out of sync.

What you need is decomposition: selling one "starter pack" reduces the three SKUs inside it. It also needs to work in reporting, so that when you ask which products are selling, the answer counts the units actually consumed rather than the bundles shipped.

Reorder points beat gut feel

Reordering from memory works until the catalogue outgrows the memory. A simple bucketed view — items that need reordering, items that are low, items already out of stock — turns purchasing from a recall exercise into a list.

Two refinements are worth the effort. First, reorder thresholds should account for stock already on order, or you will double-order every time a delivery is slow. Second, a stock valuation snapshot alongside the counts tells you what the shelf is worth, which is the number your accountant actually wants.

How Fulfillio approaches it

Fulfillio holds items, packages, suppliers and price lists centrally, and every connected channel draws on that one count. Orders decrement stock as they are pulled in from any marketplace, and purchase orders received into stock top it back up across all channels at once.

The inventory dashboard separates reorder-needed, low-stock and out-of-stock items — with reorder taking account of stock already incoming — and the stock status report adds valuation. Packages are supported with decomposition, so items-ordered reporting reflects the underlying SKUs.

Detail on the inventory management page.