Agree with the above, typically test drives are allowed, provided the viewer has purchased temporary comprehensive insurance cover.
Personally, I'd allow test drives, but would want to drive for the first 5 miles or so, with the viewer in the car to ensure the engine is fully warm before letting them loose in it. That way, I don't have to worry that they'll strain the engine before it is warm and also, they get to hear/ experience the car's characteristics when the engine is cold.
Either way, good luck with the sale, an ambitious price, but I'm sure it's merely a reflection of the condition of the car.