Look for reliable contact information. You surely don’t want to contact a guy with a fake address. Meet the buyer/seller in person before the transaction. After all, these are not yesteryear’s arranged marriages. Never send the money via wire transfer. The fraudsters cannot be tracked if the payment is made through western union/money gram. Review other/previous ads of the person. No one would sell an iPhone 4S to you for $100 (INR 5000) unless he/she has a crush on you! If it’s too good to be true, it probably is not true.
