Online auctions have become popular, leading to new research on how sellers and buyers behave in these settings. The study looks at how bidders with different risk preferences choose between first-price and second-price auctions. It finds that risk-averse bidders are more likely to enter second-price auctions if they have decreasing risk aversion, while sellers prefer first-price auctions in this case. The research also explores how bidders decide whether to participate in auctions with a Buy-It-Now option or buy at a fixed price.