But I would really bristle at being given a question like that (and luckily I don't have to interview so this will not happen).

For one think selling takes preparation, knowledge of the product and strategy. It also takes experience (for a particular product and buyer and situation). So in my mind I don't see what asking a question like that will prove other than that you can ad lib or bs in a way that doesn't matter.

Selling also takes interpreting cues (verbal and visual) from the potential buyer. With those cues you change strategy on the spot and try to hit the right hot buttons. [1]

In a hypothetical situation it's unlikely that those cues would be valid because the person you are trying to sell knows it's not a real situation.

[1] As an example the strategy that you would use to sell a car to a father for his daughter is not the same as to a middle aged woman. And by listening to the father you could tell what points are resonating and try to reinforce those points more if appropriate.