What characterizes an auction without reserve?

An auction without reserve is characterized by the fact that once bidding begins, the items being auctioned cannot be withdrawn from the auction. This means that the seller commits to selling each item to the highest bidder, regardless of the amount of the final bid, as long as there is at least one bid placed. The binding nature of this commitment encourages bidders to participate, knowing that the seller cannot back out or pull the item once the auction has started, which helps to create an atmosphere of competition among bidders.

In contrast, the other choices do not accurately reflect the nature of an auction without reserve. For instance, items that can be withdrawn any time or bids that can be canceled by the seller contradict the principle of the auction without reserve, where the seller is obligated to honor the bids as they are presented. Similarly, allowing bidders to set minimum prices is outside the standard practice for a no-reserve auction, where the auctioneer typically starts at a low price to encourage bidding.