My personal opinion would be to ask her more questions about how the animal will be taken care of. When my mom bred Chihuahuas she would refuse to sell to someone if she didn't think they could properly take care of them. I know the guy I got Yuki from seemed more interested in the sale than where the snake was going, which isn't very impressive to me... If it was an animal I took the time and energy to raise (even if it was "just a normal") and get ready to go to a new home I would want to know that it was a good home it was going to, and not going to dropped in a tank/tub/enclosure with two other snakes that were bigger than it. My suggestion would be to ask a few more questions and then decide if you really think it is a good home for your animal. If not, as a seller you do have the right to refuse to sell to her. My only suggestion there would be that if you do refuse to sell it to tell her clearly, and in the least accusatory way, why you don't want her to have your snake.