Well, Paypal is pretty safe in general to pay with. Another thing you could do is ask him for an updated picture with the date and his name in it, if he is a reputable breeder, and actually has the snake, he won't refuse. As well, ask what kind of guarantee he offers on the snakes, live arrival is a must for me, but some will offer a guarantee for about a week after you have them.
Also, check faunaclassifieds.com on the Board Of Inquiry section, just to make sure he has good feedback.
Good luck!