Car selling scam

Just making sure anyone selling a car privately is aware of this scam as my friend in Sussex has just been taken in by it.

He advertised his car for sale and was contacted by a Bulgarian mechanic who was very keen to come and view it. Two of them turned up and began to examine the car. whilst the bonnet was up the other one asked if he could see the spare wheel and tool kit was present, that took a minute or so and in the mean time the other person had closed the bonnet and wanted to start the test drive. Whilst out test driving the car started to smoke from the exhaust so they returned to the house and opened the bonnet. One of the potential buyers removed the header / expansion tank cap and pointed out to my friend that there was oil contamination in the coolant, walked to the back of the car and then pointed out there was oil dripping from the exhaust tail pipe ( they squirt this in before knocking at your door if they can and whilst your distracted add the dirty oil to the coolant)  My friend found himself in a situation where he thought his car had major engine problems and was badgered in to accepting a much lower price for the car than he wanted or it was really worth.

Please tell anyone you know selling a car privately about this scam and share it via any means you can to stop these scumbags stealing from and deceiving, honest, hard working people  like my friend who's only crime was to trust someone at face value.