A negative quantity means the system has recorded more going out than it ever recorded coming in. It is almost never a sign that stock vanished. In nearly every case the goods arrived and were sold, but nothing told the system they arrived.
This matters because the obvious fix — typing the right number into a stock count — does not hold. The count corrects today's figure, then the next sale takes it below zero again, because the original receipt is still missing.
Work out which of the three you have
1. The goods were never recorded as received
This is the common one by a wide margin. The product was added to your catalogue, it started at zero, and every sale since has pushed it further down. There is no purchase, goods receipt, or production record behind it.
How to check: open the item from Inventory → Stock Count and click its name. If the history shows only sales and no receipts, this is your case.
What to do: record what actually arrived, using Inventory → Goods Receipts → Receive. If you order from suppliers through the system, receive against the purchase order so the ordered and received quantities line up. If you are simply establishing what is on the shelf today with no paperwork behind it, a stock count is the right tool — see Counting Stock & Correcting Quantities.
2. It is a product with variations
For a product sold in sizes, colours or flavours, the stock lives on each variation, not on the parent. The parent's own number is not maintained and can drift far below zero while every variation is perfectly healthy. A negative parent is expected here, not a fault.
What to do: nothing. Count and read the variations. The Stock Count sheet lists each one separately.
3. You genuinely oversold
The item has receipts and counts behind it and still went below zero — you sold more than you had. This is the only case where the number is telling you about a real shortage.
What to do: receive the stock that arrived but was not recorded, or run a count to set the true figure. Then consider a low-stock level on the product so you get warned before it happens again.
Why we do not block a sale at zero
Selling is never blocked because your quantity is zero or negative. A queue at the counter is not the place to discover a paperwork gap, and for most stores the catalogue figure is less trustworthy than the shelf in front of the cashier. The trade-off is that quantities drift when receiving is not kept up.
The habit that prevents all of this
Record goods arriving, every time, before they are sold. It takes a minute and it is the only thing that keeps quantities meaningful. A stock count is for confirming what is on the shelf — it is not a substitute for recording a delivery, because a count leaves no record of where the goods came from, what they cost, or who supplied them.