As far as negotiating on price, you can always try. Some sellers are very firm. Some sellers almost expect haggling. If you do want to ask about a lower price make sure you do it in a polite and positive manner, because nothing shuts a seller down faster than the arrogant "I saw better pastels somewhere else for half that price!" style commentary. Since a lot of the sellers are the breeders, insulting the snakes by insinuating that they are not the BEST... or implying they are overpriced... etc.. not good.
Not saying you would... but it's somewhat accepted to point out flaws in most items when you're haggling... but when it's live creatures, it often doesn't go over well.
Examine ALL animals carefully for health/flaws and sex before you buy. And don't buy the first you see... make a round of the room first, because if you're like me, you'll find the best deal EVER and jump on it and then see a way better deal on a way better animal a few rows later on. Too late!