This is why I make part of the price, e.g. 10%, non-refundable - if the buyer flakes I can refund everything except whatever portion is non-refundable, and put the snake back up for sale. You really need it to cover yourself for the cost of feeders, time spent caring for the snake, turning away other customers, etc. while the buyer is making payments.

Personally I would reach out to the buyer one last time, ask for the remaining payment within seven days in light of the changing weather conditions, and if you don't hear from him you'll issue a refund and the snake goes back up for sale.