MADAM – Milton Silverman in A lawyer writes (ATG No 2291) sets out useful guidance on the legal status of when the hammer falls.
I am an old-fashioned ex-livestock auctioneer who eyeballs his purchasers. When the hammer falls, that is a contract in my view (and those of my age) and an auctioneer should not change their mind, unless they have made a mistake that was clearly visible.
Upholding this principle once cost me dear. I took a bid from someone who could not pay (I didn’t know it at the time) and missed a good client friend who had all the money in the world.