You need to look at the rules of the auction to determine whether you can refuse a bid and recall the item prior to the end of the auction, or even at the end of the auction. We'll look at e-bay's User Agreement, section 4, as an example. All users of e-bay agree to these terms and conditions.
Bidding and Buying.
You are obligated to complete the transaction with the seller if you purchase an item through one of our fixed price formats or are the highest bidder as described below. If you are the highest bidder at the end of an auction (meeting the applicable minimum bid or reserve requirements) and your bid is accepted by the seller, you are obligated to complete the transaction with the seller, unless the item is listed in a category under the Non-Binding Bid Policy (at Security Measure) or the transaction is prohibited by law or by this Agreement.
It seems that the answer is clear -- unless the bid is placed in a non-binding bid category, where it is clear that a seller has the right to retract the offer to sell to the highest bidder at any time, the seller is obligated to sell. For an agreement you need an offer, an acceptance and consideration paid/to be paid. In the typical instance, the seller makes an offer to sell to the highest bidder. If the auction finishes, the highest bidder has accepted the offer for the consideration bid and the agreement has been consummated. By the same token, the buyer cannot back out of the agreement since the buyer accepted the offer. It wouldn't be fair if this worked one way, would it?
In the situation of the retractable bid, the seller may state that the auction is an offer to sell to the highest bidder with the right of retraction at any time before accepting payment. Some might view this as an offer not being made by the seller but by the highest bidder. The high bidder is the offeror and the seller has the right to accept the offer by the high bidder.
What could potential liabilities be? The buyer and e-bay could sue you for any damages that occurred and you may very well be banned from usage of the system. You may also be liable for attorneys fees.
That's just my take on the agreement FWIW.