Terrible analogy. The salesman is acting as an agent of the dealership that you are buying the car from. In this case, Goat was not buying the stock from the exchange, and Nefario was not acting as an agent of the seller.
Every analogy will be different from the thing analogized, that's the point of the analogy. Can you explain why you think those differences make the analogy inapposite?
There is a world of difference between a promise from the agent of an entity selling you something, and from a random dude that happens to be nearby. Of course my analogy is bad too, but I'd say that in this case, Nefario is a lot closer to the random dude walking by than he is to an agent of the seller.