First person to bring me money takes it home.

I would NOT call the 2nd person and cancel just because there are a lot of flakey buyers out there. You'll end up shooting yourself in the foot. If a person is serious about buying, they will send you a deposit or at the very least make it up there before the other person.

Worst case scenario for you: 1st person comes to look at it and says he'll think about it and the 2nd person finds another car. This is why I always say, "First come, first served." I won't turn anyone away if they're ready and willing to buy.

I've got something to sell and whoever's got cash and ready to buy is first priority.