your not allowed to sell animals on craigslist, so any ball python found on there is gonna be a rescue. alot of time its someone going to college and can't bring his snake. or maybe just moving somewhere and can't take it. alot of times they just don't want to take care of it anymore. in that case it has probably been mistreated and in some bad husbandry for awhile. unless the person took down records it's unlikely you will have a good history of the snake.

i have also noticed that you rarely find just a snake for sale. most of the time people want to sell the whole set up they have going. most of the time the stuff they have isn't worth jack to a snake, so it really just ups the price of the snake, since you can't "sell" the animal they sell the stuff to it make up.

one last piece of advise from me... buy from a reputable breeder, not from craigslist. with a breeder you get a healthy snake from someone that cared about the animal.