©Scott Sheaffer, USA Dog Behavior
Puppy mills (also known as commercial breeders) see puppies more as "livestock" and breed them in large quantities and as economically as possible - an estimated 4 million every year in the US, which sadly equates to the number of dogs euthanized in US shelters annually. Puppy mills don't typically focus on temperament or physical well-being; as a result, the puppies these breeders sell frequently have behavioral and physical issues.
They can be so careless in how they breed and nurture these animals that I believe they represent a real threat to the future behavioral and physical health of virtually all dog breeds.
Puppy mills frequently will meet a buyer half-way between their place and the buyer’s in order to make it more convenient for the buyer (or they may offer to ship). I fear that the real reason, in many cases, that they do this is because they don't want the buyer to see their breeding facilities. Legitimate breeders not only want buyers to see their place, but they also insist on getting to know potential buyers before giving up one of their prized animals.
A puppy mill's website is usually eye grabbing with lots of great testimonials and cute pictures of puppies. Look more closely at many of these websites and you'll see that they are frequently 90% about making the sale and 10% about the dogs. Puppy mills oftentimes sell animals online too. Legitimate breeders have no need for expensive and fancy sales oriented websites.
Puppy mills generally have an unending supply of puppies. They regularly breed the dams (i.e., mothers) too early and too often which can cause behavioral and physical issues for both the dam and the pups. A legitimate breeder customarily has a very limited numbers of puppies and only at certain times of the year. Having to wait a season to get a new puppy because the breeder doesn't have any available is a good sign - one of the best signs that you are dealing with a legitimate breeder.
Puppy mills have all kinds of official sounding registrations for their dogs and most of them carry little weight. What you're looking for is AKC registered dams and sires (i.e., fathers), for example.
Puppy mills sometimes create unusual designer dogs. Designer dogs are simply purebred dogs that are mixed (e.g., Labradoodle). There is nothing wrong with designer dogs, but puppy mills may take it a step further and selectively breed unusual combinations of physically abnormal dogs. They can end up creating bizarre and unhealthy dog "breeds" that you won’t find on the AKC list of breeds and charge thousands for them.
Breeders who sell puppies to pet stores must be registered with the USDA. A USDA dealer license tells you this breeder is selling a whole lot of dogs – not typically a good indicator. A legitimate breeder would normally not sell to a pet store or breed in the quantities that make it necessary for a USDA license.
If a breeder makes 100% of his or her income from selling puppies, there is a good chance that this is a puppy mill operator. Legitimate breeders can't ordinarily produce enough puppies to make a living. They usually provide other dog related services or have another job in order to pay the bills. They raise puppies because they love a specific breed of dog.
Legitimate breeders typically focus on one breed. Puppy mills frequently offer multiple breeds.
Puppy mill owners focus almost exclusively on the appearance of the puppy. Legitimate breeders focus on the puppy's physical needs and requirements, the puppy's dam and sire, and how the puppy has been socialized.
Sadly, the majority of puppies in pet stores, flea markets and purchased online are puppy mill dogs.
Purchasing a puppy from a legitimate breeder can be a great experience in lots of ways – I personally have no issues with buying puppies from established, qualified and legitimate breeders. My Cavalier King Charles Spaniel came from a good breeder.
Puppy mill dogs can turn out to be great dogs too. My German Shepherd is a rescue and I’m fairly confident he is a puppy mill dog - and FYI, he has been one of the greatest dogs I’ve ever owned. Don’t forget that rescuing a dog is a great way to bring a new dog, even a puppy, into your home too.