Only information needed for purchase should be requested. If you're asked for personal information, there should be an explanation of how the information will be used, and to whom it will be given.
Don't conduct business with an anonymous user. Get the person's real name, business name (if applicable), address and phone number. Verify this information before buying. And don't send your payment to a post office box.
Be more cautious if the seller uses a free e-mail service, such as Hotmail, Yahoo, etc. Most people who use these free services are honest; however, most problems occur when a free service is used. After all, with a free e-mail service, it is very easy for the seller to keep his or her real identity and information hidden.
Save copies of all of the e-mails and other documents involved in the transaction. Then, if you discover that an item is counterfeit, you have documentation to help you deal with the problem.
Use common sense and trust your intuition. If you have a funny feeling about an item, don't buy it. You're very likely right that it is counterfeit.