Hi, I need help with understanding what it means to exercise reasonable skill and care as an auctioneer?

I have been given a scenario whereby the auctioneer has sold a car for a higher price than the reserve price, but the defendant is denying that the reserve price is the price she sold it for. The auctioneer has sold the car for 350,000, which is higher than the reserve price which was 340,000. The defendant has bought a claim saying that he told her to set the reserve price at 420,000, which in reality he did not. He is saying that the auctioneer did not exercise reasonable skill and care, what does this mean?