Remove yourself emotionally from the sale. I am as ethical and well-meaning as anyone when I buy or sell. If someone tells me "I'll take it" and my feeling of his character is that he means it, I will hold it for him until he gets the money or tells me he can't swing it.
If I am a bit concerned about the persons sincerity, I will say "either a deposit, or it is still for sale" up front.
In your case, the "nice guy" may be that and very sincere, but he did not say he would take it, so the airplane is for sale and you must operate under that premise.
If guy number 2 brings you the money, then you have a sale.
If guy number 1 calls you before then, you will have to decide if his word is better than cash then live with your decision if he flakes. I don't believe you have a dilema at this point since guy 1 did not tell you he wanted the airplane.
