A bid left before an auction opens, on items where the seller has turned them on.
Sealed
You can see how many pre-bids an item has, and whether yours is the top one. You cannot see an amount — nobody can, including the seller, until it opens.
That is the point. A visible pre-bid would just be an early auction with worse information.
What happens when it opens
The highest pre-bid leads at one step above the next-highest — not at its own full amount. So pre-bidding your true limit does not mean paying it.
From there the auction runs normally.
Raising, lowering, withdrawing
Raise: yes, any time before it opens.
Lower: no.
Withdraw: until an hour before a timed auction opens, or until the show it is queued in goes live. After that it stands, and it is a real bid you are committed to.
Where they are not allowed
Sudden-death items. On a short clock that never moves, a head start stops being an advantage and becomes the result — so pre-bids are off for that format. The three formats.
The same checks as a live bid
A pre-bid is checked when you place it and again when the auction opens, because an unpaid win or a block can arrive in between. If something changed, the pre-bid does not stand.
Questions people ask
Can the seller see my pre-bid?
Not the amount, and not until it opens.
Does pre-bidding mean I pay my full pre-bid?
No. The top one leads at one step above the next-highest.
Can I pre-bid on everything?
Only where the seller turned it on, and never on sudden-death items.
What if I change my mind?
Withdraw it before the cut-off. After that it is a bid like any other.
Is a pre-bid the same as a maximum bid?
They are the same machinery at different times — a pre-bid before the auction opens, a maximum while it is running.
More: how bidding works.