Read the auction carefully, if you have questions about the item ask before you bid. You may not be able to rescind the bid if the answers you receive are not to your liking. Find out if the seller is willing to use an escrow service. Escrow is when a buyer places money in the custody of a trusted third party. The money is then paid to the seller once a specified set of conditions are met.
Most auctions have an escrow service.
Consider using one if you are unsure or want extra security when using an auction. If you do not wish to use an escrow service, consider using a third party credit card service, such as Xcom's PayPal. There is a fraud insurance policy if the seller is verified. Be sure the seller is verified if you wish to use this service. Ask before you bid if this is the route you would like to take.