I have been approached by someone who wants to pay via cheque and they are offering me a price than advertised.

Cheque overpayment fraud is when a fraudster pays a business for goods or services by a fraudulent cheque. The cheque is made for a higher than the actual value.The business reimburses the fraudster with the excess amount of money that was apparently paid to it, before it discovers that the cheque was not genuine. Cheque overpayment fraud is often a method used for goods and services sold through classified adverts.