Selling in several places is ordinary and sensible. The failure is always the same one, and it is not the extra work.
The actual problem
One object, two listings, two buyers. The second sale cannot be honoured, so somebody gets a refund, a wait and a reason to doubt you. On an auction it is worse, because the person who won spent time deciding to.
Everything below is about preventing that one event.
Three ways to handle it, in order of how well they work
Split the stock. A shelf for here, a shelf for there, and nothing crosses over. Least clever, least error, and what most people end up doing.
List in small batches. Only what you can personally keep track of is live at any moment. Slower, but you always know.
One master list, updated the moment anything sells. Works if you are disciplined, and the discipline has to be immediate — "I will update it tonight" is how the double sale happens.
What to keep in one place regardless
Your descriptions and measurements. Write them once and reuse them. Different descriptions of the same object in two places is how a return starts.
Your delivery pricing. If one place charges a buyer less than the parcel costs you, that difference is coming out of your pocket quietly.
When one platform is the right answer
If your stock is small, specific and does not take long to sell, splitting it across two audiences makes both look thin. A shelf of five things is better in one place.
Questions people ask
Does Metzee mind?
No. Nothing here asks you to sell only here.
What if I do oversell?
Cancel and refund at once, say so plainly, and expect it to cost you some standing. The app refunds the buyer; the apology is yours.
Should a live show use the same stock as my listings?
It can, as long as a show item comes out of the listings the moment it sells in the show. That is the same rule, indoors.
More: keeping stock straight, if you sold something you cannot send, bulk listing.