Message the buyer now, before deciding anything. A seller who says "I cannot send this, here are your options" on day one keeps most of the goodwill. One who goes quiet and cancels on day five does not.
How it happens
The same item sold twice — in a show and as a listing, or across two shows.
Stock not updated after a market or a previous sale.
It broke, or cannot be found.
A bulk import put the wrong stock number in. My bulk import did not work
None of those is rare and none is shameful. The handling is what matters.
Your three options
1. Offer an alternative. Something similar you do have, at a fair adjustment. Works more often than people expect.
2. Ask them to wait, if you can genuinely get another and they are willing. Give a real date, not "soon".
3. Cancel and refund. Clean, and the right answer when the other two do not apply.
Let the buyer choose. It is their order.
What cancelling costs
It counts against you. Seller cancellation rate is one of six measured standards and has the tightest thresholds of any of them. One is recoverable; a pattern is not.
No fee is earned — a fully refunded order is not a sale — so there is no version where this is profitable.
Cancel early rather than late. Day one is forgivable; day six, after the buyer has waited, is the review you cannot undo. The seller cancelled my order
What not to do
Do not send something else without asking. That is a not-as-described case you will lose.
Do not stall. Silence turns a cancellation into a complaint.
Do not relist it while the buyer is waiting for an answer.
Stopping it happening again
Keep stock accurate, especially across a show and your listings. The duplicate-sale version almost always comes from a show item that was also listed.
Use daily capacity so a busy night cannot outrun what you actually have. How to set daily capacity
Mark items off sale before a market, not after.
Check your lineup against your stock before going live. Two minutes, and it prevents the worst version of this — selling the same thing to two people in the same hour.
More: I broke it while packing, the seller cancelled my order, how refunds work.