Police officers are appealing for witnesses after the BMW 5 Series was stolen from Greenhills Road, Northampton.
The incident happened between February 26 and February 27 when a BMW 5 Series was taken from the area following a test drive that day.
It is believed that the potential buyer had swapped the spare key during the test drive and returned overnight to steal the vehicle.
Anyone who saw the incident, or has information about it, is asked to contact Northamptonshire Police on 101. Information can also be shared anonymously via Crimestoppers on 0800 555111.