Alot of people don't have $500 or $1000 just laying around to buy a new snake with. If they are in the process of saving for a snake and then they fall in love with one they see it gives them the opportunity to put a deposit on the animal and continue to pay for it so someone else doesn't buy it. Or if they know they will have some additional income coming in the near future(tax returns).
As a seller it makes perfect sense to offer payment plans as long as the terms are understood by both sides and the buyer doesn't want some crazy long term plan.