If you are getting it shipped and it isn't a known breeder, ask if they are certified to ship snakes, and what companies they are certified to ship through.

Ask for photos of snakes you want (Adam from 8-ball, the only time I have ordered online, sent me about a half dozen extra pictures of the snake without me even requesting them).

If you want to be thorough, ask for pictures of their setup, and ask for contact information for their personal snake vet (in other words a reference).

If the snakes you are looking at are like 200 dollars or more and the person is unknown, go through all that with them if you need to ease your mind. Most people won't mind because none of this is too much effort on their part.

Don't forget to quarantine any snakes you get, and make sure you know what the snake is eating, and that it has been eating.