Just like the others said, look out for mites, ask for the feeding response of the snake, if it eats frozen or live, take a look at the mouth and nose just in case you don't happen to see any mucus coming out (really improbable), and keep a visit card of the seller, if he doesn't have on make sure to at least have his e-mail or other way to make sure you can contact him just in case the worst happens.
Other than that, make yourself A BUDGET, how much you want to spend on the snake, because its going to have a cost to keep it aswell. If that's not a problem then choose the morph you'd like to admire even when it grows (as some tend to change their initial colors that had as babies) or the morph you've always liked!
I hope you will have fun choosing your new snake