Decide in advance how much you want to spend.
Make sure you have inspected everything carefully before bidding, as you may have limited redress.
Read the catalogue and sale notice small print carefully.
If you are buying something expensive, such as a car, take someone along with you who has some experience.
If you have a problem selling something you bought at auction, you have to take the matter up with the seller, not the auction house.
If something is described as "sold as seen" you have no redress.