Could be the 'nick to order' scam i.e. they cut and paste details of a car for sale out of Autotrader into eBat, then put a stupid price on it.
They go and view the car with the genuine seller, get to see inside their house and scope how to nick the keys. When it sells on eBay, they pay the genuine seller a return 'visit' and nick the car.
A friend of a friend had this happen to him with an M3. It was only when a genuine buyer asked him why his car was being advertised on eBay for £5k less than he had it up for in Autotrader that he worked out something was going on. Spoke to police and apparently it is a regular scam.