It is important that if you are a high bidder, that you contact the seller right away and tell how you are going to pay. Also, you will need to tell the seller your address. Send your payment promptly and make sure that your check is good and that your credit card is valid. Obviously, civility is important in the tone of all your communication. The person you are contacting will be much more willing to work with you if you are civil with him or her.
Many online auctions have ways to discipline errant participants.
Some will suspend members who disobey the rules. Don't be afraid to report abuses. Online auctions can be both a source of fun and bargains. Participate with the proper cautions and you may have a great experience as well. Learn how to buy on them without getting burned.