One item, one listing, one quantity. It stays simple until you are also selling at a market, on another platform, or out of the same cupboard your family shops from.
Quantity
A listing can hold more than one of the same thing. In a show, that changes the behaviour — multiples suit a fixed price better than an auction, because auctioning identical units one after another drains a room.
Where it goes wrong
The same item in two places. Sold at a stall on Saturday, still listed on Sunday.
Made-to-order treated as stock. If it takes you a week to make, selling four in an hour is a promise you have to keep.
The one you kept back. Easy to list by accident, hard to explain.
If you do sell something you cannot send
Tell the buyer immediately and cancel. It costs you a strike and it is the right thing to do. Do not send a substitute without asking — that is a dispute, not a solution.
How cancelling works covers the other direction too.
The habit that prevents it
Pull the physical item before the show and put it on the table. Anything not on the table is not in the show. It sounds too simple to matter and it is the thing experienced sellers actually do.
Questions people ask
Can I limit how much I sell in a day?
Yes — useful for made-to-order and for anything perishable.
What if two buyers win the same item?
They cannot. Quantity is held as it sells.
Can I relist something that did not sell?
Yes, and change the start price before you do.
Should I list everything or hold some back for shows?
Hold back what benefits from being explained. List what does not.
Next: your first live show.