The vendor accepts the offer, then finds the buyer sends a cheque for more than the asking price. The buyer then asks for the excess to be returned to them.

In the same way as the Nigerian 409 scam relies on the victim being willing to be involved in something illegal, the conman relies on the vendor realising all is not quite above board.

The willing victim will often turn a blind eye to get the full asking price, or more.

The fraudster's cheque appears to be legitimate, but is often drawn on a non-UK bank.

The cheque will be a forgery, says the Marine Unit.

The fraudster will pester the recipient to forward the balance by instant telegraphic transfer system as soon as the cheque shows as cleared funds. And here's where the scam succeeds.

The bank clearing system will often show a cheque as cleared after three days. If the seller returns the excess funds on that third day - as the fraudster insists - a day or two after that they'll get a call from the bank telling them the cheque is worthless.

The victim is out of pocket and has not sold the boat.

According to the Police, many victims are reluctant to report the fraud as they are aware they were willingly participating in money laundering operation.

"As it's unlikely this situation would be covered under your insurance, we would urge people to be cautious when selling a boat, " said Joe Field, marketing manager for Navigators & General, part of the Zurich Group.

The chances are that if someone offers more than the asking price, it is probably a con.