auction 'buy backs'

[NovaMob03]

I've heard that the about 40% of the cars sold at auction are bought back by the owners. How does this work when a bidder has acted in good faith and placed the winning bid? Do bidders sign an agreement allowing the owner to buy it back if it doesn't meet what they think it's worth? Is there compensation paid to the winning bidder? Does the seller pay the auction both the buyer & seller's cut? Also, what if the buyer feels the car he just won was misrepresented and wants to back out? Would he be forced to keep it?
Using John O's Gibb Nova for an example. If I were John, I'd want to buy it back, but if I were the winning bidder at 150K, I'd fight to keep it. Who wins?