"Owned one for a week and then it ran away" If he doesn't know that BPs aren't poisonous or venomous, I doubt he had one at all... even for a week. This sounds like the type of person that's going to plop it in an empty aquarium and forget about it. Good on you for following your instincts on not selling, but still being courteous and educating the best you could.

Perhaps what you could do is tell him that you will sell him the snake if he can show you proof that he bought an enclosure and the proper equipment? It might turn him away, but if it does, that's not the person you want to sell to anyway.