In a real live auction, the vendor can withdraw a lot at any time, even while bidding is progressing.

The auctioneer would have a claim against the vendor for costs but the bidders have no recourse.

I used to go to live London auctions every week. This happened a few times.

On ebay - if the auction runs its course or is ended early in favor of the high bidder then it is a contract. Otherwise it is not.