Well it needs to be a little more diplomatic like I do not think this animal is the right fit for you......works wonder without being insulting and it covers a variety of thing from idiot you know have no business owning a snake to people wanting to buy a young hatchling for a 10 years old.................Always worked for me.But I guess it would be hard to deny someone. "I'm sorry, you're too stupid, I can't sell you this animal" probably wouldn't go over well.
100% this I don't care what animal you sell as a breeder, a conversation to gauge whether or not the animal is right for the person goes a long way not only that, but if the person is a forum member or has a facebook, it can tell you a lot as well.And to be fair, I think a basic competency test should be required for ALL animal purchases. They are living creatures after all.
Sure sometime you have no red flags I have seen people going from very passionate to neglect in matter of years but I do think some breeders make very little effort to gauge customers and they should especially with things that have the potential to bring some real bad publicity on all of us. (giant, venomous etc)