If possible ask if you can handle them before you buy, also ask if they're good feeders, and if its feeding time soon if there's one you're interested in buying ask if you can see it feed. Nothing worse than a bad feeder.
Also ask as many questions as you can
What is it being fed (live / thawed frozen) (pinkie 2x pinkie etc)
How old it is (when was it hatched)
Parents (for breeding purposes if youchoose to breed in the future)
Size of parents (generally speaking large corns will give birth to large corns, and small ones to small corns)
Also ask for recommendations on things like bedding, how often to feed, temperatures, size of enclosure you'll need etc. These are all things you can find out online or in here, the reason I say ask is because some breeders have no clue what they're talking about. I've seen one recommend using the eco-earth stuff which is for high humidity snakes / reptiles and would probably cause problems for a corn. I've also seen them say things like "you'll need a 30 gallon tank" when its still a hatchling (a tank like that they'd get VERY stressed, and would probably escape easy)
If they give bad answers or have trouble answering look elsewhere.