I agree that there are many benefits to letting a puppy stay with its mom longer than most breeders allow. I don't think 6 weeks is the age to sell/send pups off to a new home. Puppies get most of their social skills by 12 weeks of age, and mom and the breeder are their teachers. Mom teaches the puppy dog behaviors and skills, and the breeder should teach human interaction to puppies.
But, not everyone knows how, or care to socialize a young puppy. Take my dog Cheyenne, she was 12 weeks old and with her mom and remaining sibling, when I met her. But she had never left her concrete pen, or been touched by a human. When I touched her she screamed like I was killing her. She also was severely emaciated, had eye infection, and was a very sick pup. I purchased her, called an animal abuse hotline, and then took her to a vet. She is now healthy in body, but not in mind. She will suffer the rest of her life from the lack of socialization. I could not re-home her even if I wanted too, she would end up being euthanized, as she either pees herself in fear or turns to fear aggression around strange dogs and humans. I work with her every day and have her in obedience class as well. She has made progress, she no longer cowers behind my knees.
It takes more than just the mom to make a well balanced puppy. I have seen some puppies go to new homes at 8 weeks old and be fine, and others that end up having problems. It's not just the age, it's the handling and interactions the puppy has that determines how it will end up. Phantom was well sociliazed by his breeder, and I continued his socialization when I brought him home at 5 weeks of age.
So when buying a puppy, it's not just the age at which they are being sold, it's how well their breeder socialized them. Along with looking at OFA, Cerf, kennel conditions, etc, check how the puppies react to people and other animals, before buying. A dog with no social skills needs a lot more of your time and a different way of being handled. Fear aggression is actually the worst type of aggression, and most people end up putting this type of dog in shelters or euthanizing them.