If possible, use a credit card. It offers the most protection if there's a problem. Consider using an escrow service if the seller doesn't accept credit cards. Know the Seller - Find out who you're dealing with. Verify the seller's identity before you place your bid, and be wary of sellers who you can't identify.
Some auction sites post feedback ratings of sellers based on comments by other buyers.
Before you bid, find out what form of payment the seller will accept. If the seller accepts only cashier's checks or money orders, decide whether you are willing to take the risk of sending your payment before you receive the product. Generally, sellers specify the cost of shipping the item and give buyers the option of express delivery for an additional fee. If you're uncertain about shipping costs, check with the seller before you bid. Check on the seller's return policy.