I'm not in the position of actually doing it yet, but I can say that every single seller I've looked at or contacted has had some type of payment plan, typically no more than 60 days, with several at only 30 days. All have been 20-25% down, a couple requiring a minimum of $100 down, but not many. All have had very explicit terms of services relating to their payment plans, including payment dates. I do distincly remember seeing one that offered 90 day plans only on purchases in excess of $1000, so you could do 4 payments.
So far, I intend to do something similar to a mashup of all these options: 25% down with a $50 minimum, 30 day plans as standard, 60 day plans over $500 and 90 day plans over $1000. All payment plans must be worked out and agreed to prior to a deposit being made. Payments must be made at least once per calendar month. If a single payment is more than 7 calendar days late without contact from the buyer(preferably before the deadline passes) I will notify the customer that they have 3 business days to contact me or their deposit will be forfeit and their payments in excess of the deposit will be held as store credit for no more than 30 calendar days before it too is forfeit. If that store credit is used for a deposit on another snake, it will be non-refundable in it's entirety, regardless of the percentage. No interest, no fees for payment plans.