In a classified advertisement scam, scammers respond to an advertisement for anything that is being advertised at a reasonably high price (for example a car, a computer or a snowboard). There are various variants of this scam; typically, scammers, after an initial phase of feigned interest, agree to buy the item and offer to pay for it with a cheque with a much higher value than the agreed price, using various excuses. The scammer will ask to have most of the difference paid back in cash at time of collection, supposedly leaving the rest to the victim as a reward for their flexibility and inconvenience. The collection will be arranged soon after the money will be made available in the victim’s bank account. The victim will not realise that having the funds available is different from having the cheque cleared, and therefore will happily agree to the terms. The cheque clearing process can take weeks, after which the bank will claim the whole sum back because the cheque is fake.
This is also used over the IP Relay. There is a case where the scammer requests a Driver’s License or International Passport be faxed over as he represents a close friend of his who is dying.
Source : wikipedia