That's why you want to go to a reptile show with a general idea of what you're looking for and what it's value is. Know what a quality specimen looks like and that people will generally ask more for a cleaner, brighter, larger, etc animal. Some sellers will outright offer you a lower price if they see you staring at something for a while. Otherwise, make a reduced offer, I've gotten away with a 16-33% reduction. Some buyers will make especially good deals if you're getting two or more animals. Show that you're really interested in the animal and that you appreciate it's appearance. Don't start pointing out flaws on the snake or you'll just insult them on their stock.