We've had many families who have needed to purchase a new violin after realizing that the violin they bought on Craigslist could not be used. Here are some questions to ask the seller of the violin.
How old is the violin?
Was the violin ever stored in the car or outside on a hot day? The expanding and contracting of wood increases the chance for cracks and damage to the varnish.
Look for cracks and dents on the violin.
Be sure the violin is made out of solid wood, otherwise the violin will likely break within three months.
Check for the condition of the components (ie. strings, pegs). Are the pegs loose? Are the strings starting to rust?
Check for the condition of the case. Are the zippers in good condition?