Selling at auctions, you set a reserve price, a price below which you will not sell. That is either a fixed reserve of a set price and no less, or a discretionary reserve, whereby the auctioneer may sell for slightly less than the price. That makes sense sometimes; if you set a reserve at £300, 000 and the bidding stopped at £299,000 you might feel a bit silly for losing the sale, which you would do on fixed reserve.
The buyer at auction has to pay a deposit and then the balance within a couple of weeks. That isn't much time to find a mortgage! Buyers do not necessarily have cash in hand when they buy; some have finance available already because they know the mortgage lender has agreed in principle and will lend enough on the type of property they are buying, in the condition it is. The great thing about auction is that the buyer is legally bound at the fall of the hammer; there's no sudden lower offer or buyers backing out .