Say it, straight away, clearly. "Sorry — I said two thousand, the start is one thousand." The room forgives a correction instantly and remembers a quiet one for the rest of the hour.
What you are actually bound to
The listing, not your speech. The start price, the reserve and the buy-it-now are what you set on the item, and those are what the auction runs on. Saying a different number aloud does not change them.
So a misspoken price is a communication problem rather than a contractual one — as long as you correct it.
What you cannot undo
A bid. Once a bid is placed it is binding on the bidder, and you cannot remove it. I bid by mistake
The settings, once it is running. An auction's settings lock at the first bid and when it goes live: This auction is live, so its settings are locked. So if you set the start price wrong, you cannot fix it mid-item.
The four mistakes that actually happen
1. Wrong price said aloud. Correct it out loud. No real harm.
2. Wrong start price set on the item. Harder. If nobody has bid, remove it from the queue, fix it, and queue it again. If somebody has bid, you have two choices: let it run and accept the price, or let it close and relist. Do not pull a live item with people bidding on it — that does more damage to the room than selling one thing cheaply.
3. Described the wrong item. Correct it immediately and show the right one. An item sold on a description that belonged to something else is a misdescription, and it comes back for 7 days whatever your returns policy says. Fixing it on camera is much cheaper than a return.
4. Said something is included that is not. Correct it before the item closes, and say it twice. This is the one most likely to turn into a dispute, because the buyer heard a promise. Something is missing from what I bought
The rule that covers all four
Correct it before the item closes. Before, everything is recoverable. After, you are dealing with a buyer who heard one thing and received another.
And if an item closes on a promise you cannot keep, say so to the winner immediately and offer the fix — a partial refund, or cancelling before you ship. A problem you raise yourself costs far less than one the buyer discovers. A buyer is asking for a refund
What the chat is for
Say the correction in the chat as well as aloud. The chat persists for an hour after the show; your voice does not, because shows are not recorded. If something matters, write it. Is the show recorded?
That is also the answer when a buyer later says "you said on camera that…". A sentence in the chat is a record; a sentence on air is not.
Do not stop the show
The instinct after a fumble is to apologise at length. Correct it in one sentence and move to the next item — a room loses interest in a seller apologising far faster than in one who made a mistake.
More: what to say on camera, how to put an item up during a show, can I practise before going live for real.