The puppy should not leave the home/kennel before 8 weeks, many breeders are electing to keep the puppies up to 12 weeks. Some States prohibit the sale of a puppy prior to 8 weeks. The period between 4 and 8 weeks is a crucial  where the puppy learns to interact with littermates and mother. Puppies have been known to elicit aggressive and other behavioral problems if taken away from their mother and littermates too early. Please note that is does not relate to the age that the dogs is weaned. The period between 6 and 12 weeks the puppies should be introduced to people and other animals. Your puppy should have received at least one vaccination before being sold. If not, there's no guarantee your puppy will not have or soon contract a life-threatening disease.