Do they have a health guarantee? If so, you have no worries.
However, if you are serious about buying the snake tell them you want it. During the purchase you would certainly get to hold it and check it out. Im sure if they are really that serious about their animals the snake is in good shape and will pass your inspection.
I don't see why they would refuse you to inspect the animal at the time of purchase. I can see how they don't want the tire kickers constantly holding their snakes and putting them through a lot of human interaction just to please some people that think holding a snake is cool and don't have any interest in actually buying one.