Generally, the bids are the offer which the auctioneer may, but is not bound to, accept. (Chitty on Contracts, § 4-025). Many Sale of Goods Acts specify that the "sale is complete when the auctioneer announces its completion by the fall of a hammer" (see Ontario's Sale of Goods Act; see also Chitty on Contracts, § 4-025). If the auctioneer is not going to accept the bid, the auctioneer may withdraw the lot.
If the property is put up for auction subject to a reserve price, no contract is formed if the auctioneer mistakingly purports to accept a bid lower than the reserve price (Chitty on Contract, § 4-026). As for how this should play out in practice, see Halifax, City of v. Miller and Johnson Auctioneers Ltd., 40 N.S.R. (2d) 35: "the proper course for the auctioneer... was to make known that there were reserve prices at the auction and if in the case... the reserve price was not reached to withdraw the unit from the auction," citing Halsbury's Laws of England (4th Ed.), vol. 2, para. 747, at p. 383.
Any after-the-fact declaration of the existence of a reserve bid has no bearing on the formation of the contract.
The only thing that matters is whether the property is put to auction subject to a reserve price.
- If it is not put up subject to a reserve price, then the contract for sale is formed with the highest bidder when the auctioneer announces the completion of the auction.
- If the property is put up subject to a reserve price, then no contract is formed, even where there is purported acceptance by the auctioneer.