Being able to advertise and purchase items online is very convenient and has become somewhat commonplace. Along with these advances in technology, however, comes the potential for fraud.
One scam that is becoming increasingly more common involves a fake "buyer" who pretends he is interested in purchasing whatever item you may have for sale. He sends a "cashier's check" for an amount that is more than the purchase price and then asks you to wire him the extra money back. The "cashier's check," of course is a fake. If you happen to deposit it - whether or not you wire the "buyer" his excess funds back - you will be responsible for making good on the check when it is returned to your financial institution as counterfeit.
You will not be able to verify every buyer interested in what you have for sale, but you can be cautious. No reputable buyer is going to offer you more than what you are asking for an item, nor will he send you a check for more than the requested purchase price.