Is it a good deal? This depends on the quality of the animal. Its it eating rats? Does it have a strong feeding response? Does it have a healthy girth to it? Does it have good colors? Are you sure its a female? Are you going to be able to check? Does it have mites?
These are all questions you should ask yourself before buying a BP off craigslist.
It's not impossible to get a good deal off CL, but I believe the odds are against you.
Also I'd recommend checking out this article: http://ball-pythons.net/forums/showt...ple-s-Problems