This is a similar scam that was happening on Craigslist a few years back. They would offer people large amounts of money for items for sale. For instance a $500 item...they would sent a cheque for $1000 and say that they will pay for the item but the seller will need to forward the remainder of the cheque to a third contact. The person cashes the cheque and then forwards the extra money but in the following days the bank will claim the cheque was a fake and the person who cashed it be responsible to pay them back.
Local authorities will take interest in these cases if you can convince someone to deliver the cheque in person. This is very unlikely as the fraudsters are aware of this method of getting caught.
Best thing you can do is email them from a secondary email address and tell them to go to someplace you would like to see them end up in your own choice of words. Be colorful.