It is not clear from your post if you have already placed a bid and want to cancel it or if you are asking because you want to know before placing the bid.

Here are the general rules:

https://www.ebay.com.au/help/buying/bidding/retracting-bid?id=4013

Or, as you posted under buying, if you mean make a bid rather than take a bit

If there is another time left, retract the bid

If you can’t retract, you can’t cancel

Dont bid on things you don’t want

If you win the auction, you will need to pay

you need to be clear in what you are asking.

Are you a buyer who made a bid and want to cancel/retract that bid?

Are you a seller who has an auction that has attracted a bid and you wish to cancel/end the auction?

Are you a seller who had an auction end with just one bid, at an amount you are not willing to accept?

Let us know a bit more and you will receive advice.

Cheers