Hymie is spot-on.
Whoever has kicked this claim out does not understand how their own policy wording works.
The following is the verbatim text from the Chartered Insurance Institute Motor Insurance exam course book;
"The clause states "you are not insured for loss of your car be deception by someone who claims to be a buyer or a buying or selling agent.
Some insurers would not place too strict an interpretation on the above exclusion. The intention may be to avoid those claims where a policyholder fails to take reasonable steps to protect the property at the time of purchase.
Consider the following example
A vendor leaves the keys in the vehicle, while demonstrating the car to a purchaser, and grants a thief an early opportunity to abscond with the vehicle.
In this instance, mere inadvertance on the part of the policyholder would not enable the insurer to succeed with the deception exclusion"
Unless there are fact we are unaware of, the loss of your car is insured - your policy will have a complaints procedure which must be followed before your can contact the FOS.
First of all, have the claim referred to a manager - if you still get no joy, invoke the complaints procedure. If still no joy, got to the FOS.