How to Avoid Overselling Your Bestseller

24 May 2026 · The Glow Trade

A creator mentions a sunscreen on a Sunday evening. By Monday lunchtime the small shop that stocks it has taken orders for three times the units it owns, because the product page kept accepting orders at zero and the same dozen units were also listed on a marketplace. The rest of the week goes on refunds, apology emails and a supplier who has just been asked the same question by every other account it has.

We see this from the supply side several times a season, and demand is rarely the problem. The shop had no rule for when to reorder its best line, and no rule for what the website should do when the answer is none left.

The short version

Directive 2011/83/EU, Article 18(1), obliges a trader to deliver not later than 30 days from the order unless a different time was agreed; once that period is missed the customer can set a further deadline, cancel and claim a full refund.
Directive 2005/29/EC, Annex I point 5, bans in all circumstances inviting customers to buy a product at a price without disclosing that the trader has reasonable grounds to believe it cannot supply it in reasonable quantities, which is what a live buy button on an unsourced line amounts to.
A working rule for a bestseller in a small online shop: reorder when stock falls to two lead times of sales, with lead time measured on your own last three orders from order date to shelf date, using the longest of the three rather than the average.

Where the phantom units come from

Three mechanisms, and a shop usually has two of them running.

The first is a setting. Most shop platforms have a switch that keeps a product purchasable when tracked stock reaches zero, and it is easy to leave on when listings are imported or duplicated. Turn it off on every line, then turn it back on only where you hold a purchase order with a date on it.

The second is one shelf sold in two places. Own website plus a marketplace, or website plus a counter, with stock synced on a schedule rather than live. A quiet line never exposes the gap; a line selling 30 units a day exposes it inside an hour. If you sell on more than one channel, reserve a fixed number of units for each and let the sync run underneath. The listing data itself is a separate job, covered in the product data you need to list K-beauty online.

The third is counting inbound as stock. A unit on a purchase order cannot be shipped to anyone, and a Korean order in particular has stages where the date moves. Book inbound as available on the day it is on your shelf and scanned, and nowhere earlier.

Measure lead time from your own orders

Lead time, for this purpose, runs from the moment you place the order to the moment the unit is sellable on your shelf. That includes the supplier's own processing, transit, customs where there is any, and your own receiving and labelling. Take your last three orders for the line and write down the real number of days for each. Use the longest.

Direct from Korea the transit alone is a range. Sea freight from Busan is thinly served: Hapag-Lloyd's NE2 rotation effective 5 March 2026 does not call Busan at all, so Korean boxes commonly transship and a missed connection adds a week. Air moves in days but is billed on volume: Korean Air Cargo's tariff counts 6,000 cubic centimetres as one kilo and charges whichever is higher of actual and volumetric weight, so a rushed reorder of a light boxed product pays for the carton. From a wholesaler inside Europe the same three-order measurement applies, and the answer is usually short enough that a modest buffer covers it. Whichever route you use, the second number to write down is how often you can afford to order, because that review cycle sets how much you order, and the lead time sets when.

The reorder maths for one line

Take a bestseller doing 4 units a day and a measured lead time of 12 days. Lead-time demand is 48 units. That is the stock you will burn while the reorder is in transit, so the reorder has to go out while you still have 48.

Now add the buffer, and for a bestseller our rule is another full lead time of sales, so the reorder point is 96 units. The reason for doubling rather than adding a fixed 20 per cent is what actually goes wrong. Demand on a K-beauty hero line can double overnight after one post, and suppliers slip most often on exactly the line everyone is asking for. Either event on its own eats half the buffer. Both together, which is the normal case, eat all of it.

Order quantity is the review cycle multiplied by daily sales: ordering every 14 days at 4 a day means 56 units per order, and you will be placing that order while still holding around 96. That looks heavy on paper until you price the alternative, which is a week of a dead product page for the line that brings customers in the first time.

Do not apply the rule to the whole catalogue. On a line selling one unit a week the cost of a stockout is the margin on one unit, so the buffer there is zero and the money belongs in breadth, the argument made at length in how much inventory you need to open a K-beauty store. Double cover for the top ten lines, none for the tail.

There is a ceiling as well. Article 19(1)(c) of Regulation (EC) No 1223/2009 puts a minimum durability date on the pack, or the period-after-opening symbol where durability exceeds 30 months, and that clock started at the factory. Cap stock plus open orders at what the line sells in three months, however cheap the freight looks.

When the line is gone upstream

Sooner or later the bestseller runs out where you buy it, and often at the brand as well. Whether stock can be checked before ordering, and whether pre-orders are possible, are among the questions retailers put to wholesalers most often in published FAQ pages. Why Korean hero lines disappear for a month at a time is covered in why K-beauty bestsellers go out of stock. What to do on the day:

Take the buy button off unless you hold a written inbound date. Article 6(1)(g) of Directive 2011/83/EU requires you to state the delivery time before the order is placed, and a date you cannot name is a date you cannot state.
Where you do hold a date, sell it as a pre-order with that date on the page, and only up to the quantity confirmed to you, less ten per cent for short shipments.
Cap the quantity per order at two or three units. A viral line draws resellers, and one order for 40 units clears the allocation your regular customers were going to buy.
Ask the supplier for an allocation and a date in writing. "Soon" is a word you cannot put on a product page. The email below asks in five lines.
Do not fill the gap from an unknown seller who suddenly has stock to spare. That stock exists for a reason, and spotting grey market and counterfeit K-beauty explains what the reason usually is.

The rule card and the email, to copy

Fill in the numbers once a month, for the top ten lines only.

BESTSELLER STOCK RULE  (one card per line)

Line: ____________________   Date: __________

A  Daily sales, last 28 days / 28               = ____ units a day
B  Lead time, longest of last 3 orders          = ____ days
C  Lead-time demand  (A x B)                    = ____ units
D  Reorder point  (C x 2)                       = ____ units
E  Order quantity  (A x days between orders)    = ____ units
F  Ceiling  (A x 90)                            = ____ units
   Stock plus open orders never above F.

Website at zero stock: OFF, unless a dated purchase order exists.
Per-order cap while stock is below D: 3 units.

Subject: [Line name] out of stock, allocation and date

Hello [name],

[Line name], [size], EAN [number], shows out of stock on your side.
We sell about [A] units a day of it.

Could you tell me the confirmed restock date and the quantity you can
allocate to us against it? If a date is not yet known, please say so
rather than estimate, and I will take the line off sale until it is.

If a sister size or format is in stock now, please tell me which and
how many.

Thanks,
[Your name, shop, customer number]

Questions buyers ask

Should I let customers order a product that is out of stock?

Only with a delivery date on the page that you got from the supplier in writing, and only up to the quantity confirmed to you. Without an agreed date you are inside the 30-day default of Article 18 of Directive 2011/83/EU from the moment the order is placed, and a Korean restock rarely respects that. A notify-me button loses no customer and creates no obligation.

Can I sell the same stock on my own shop and a marketplace?

Yes, if stock syncs in both directions and you keep a reserved quantity per channel that the sync cannot touch. The reserve has to cover the sales you make in one sync interval on your fastest day, and on a viral line that is more than most people set. Marketplaces also penalise sellers for repeated cancellations, a cost the reorder maths does not show.

My supplier says the line is on order but gives no date. What do I do?

Treat it as no stock. A wholesaler holding a dated purchase order with the brand can say the date; one that says soon is waiting like you are. Ask in writing, put the answer on your reorder card, and until a date exists sell the alternative you do hold rather than a promise.