Most of a listing can be changed while it is on sale — the description, the photographs, the delivery settings, the stock.
What you cannot change is anything that would move the ground under somebody mid-transaction.
Three things an auction refuses
Each one is a message rather than an opinion, and each is because an auction is a different object from a buy-now listing:
Scheduling. An auction is scheduled on the auction itself, not on the listing.
Offers. Auctions do not take offers — buyers bid instead. Can I negotiate with a seller?
Options. An auction sells the one item, so sizes and variants belong on a buy-now listing.
What not to change while an auction is running
The description, materially. People have bid on what it said. Correcting a typo is fine; changing "no chips" to "small chip at the rim" after bidding has started is not — it changes what everybody bid on.
If you discover a flaw mid-auction, say it in the chat and end the item rather than quietly editing. The room will respect it, and a bidder who finds out afterwards will not. What if the seller lied about the item?
What is safe to change any time
Better photographs. Always an improvement, never a misrepresentation.
More detail — measurements, materials, what is included. Adding information is the one edit that only helps.
Delivery settings, though a buyer mid-checkout sees what was shown to them.
Stock, on a buy-now item with several.
What you cannot do at all
Change a sold item. It has sold. Duplicate it to sell another one — the app says exactly that.
Relist something Metzee took down. The block stays until it is lifted. Why was my item taken down?
Change the price of an auction once it is running. The price is the room's.
The honest rule
Adding is safe; redefining is not. If the edit would make somebody's existing bid or basket about a different object, do not do it quietly.
If you listed it wrong
Before any bids: fix it, or take it down and relist properly. Cheap.
After bids: say so publicly and end it rather than editing. A cancelled auction with an explanation costs far less than a sale somebody disputes. Can I refuse to sell if the bidding ends too low?
More: how to write a good description, my listing is not showing up, auction or buy now.