All states in the United States require puppies to be at least 7 weeks old before they are sold. The American Kennel Club recommends a minimum age of 8 weeks.
The age at which a dog may be sold is attached to the age of weaning. Most breeds are completely weaned by 8 weeks old, but smaller breeds are more delicate and require a longer time with their mother. Any puppy being considered for sale should be comfortable eating independently of its mother. It is not only illegal but also unethical to sell a dog that still relies on its mother for nutrition.