I don't think its illegal or unethical if, and only if you tell the authorities and the potential buyer. Its called a rebody. Thats all the good bits put on a different body and/or chassis.
Selling a car as a X when it is only the plates from said car, is illegal. Its fraud. There was a case in Australian Muscle Car resently where a car advertised and sold as something, was not infact as it should and it got ugly and ended in court.