There is a difference among a puppy mill, backyard breeder, and irresponsible owner, in my book.
Puppy Mill: produces large quantities of several types of dogs for the sole purpose of resale. Dogs are often “popular breeds” such as Yorkies, Maltese, Havanese, etc. and are pumped out like factory-made items. Their prices are generally MUCH lower than a quality/healthy dog as in $400 for a “purebred Yorkie.” The dogs are rarely registered with a reputable organization and are registered with one that anyone can use (Continental Kennel Club, here in the U.S.)
When you go to a puppy mill, one of the first things that you will encounter is the smell followed by the noise. The smell is overwhelming because the dogs aren’t necessarily kept in glorious kennels. The smell is either urine, feces, and rotting flesh, or bleach so your nosehairs are burning. The sound is deafening.
The dogs are skittish with minimal human interaction. Sometimes they are ill or have skin issues.
The seller won’t ask any questions besides your name for a bill of sale and how you will be paying for the animal. I went to NW Georgia to a location that was selling Cavalier King Charles Spaniels for a friend. I thought it odd they were $300. I showed up and wanted to cry. Literally HUNDREDS of dogs. There was a “breeding barn” with metal cages of dogs shoved in them, stacked and mounted on the wall. There were kennels upon kennels made of old chicken houses. The smell was AWFUL.
Backyard Breeders: Sort of like puppy mill on a smaller scale.
Irresponsible breeders: Whoops! My dog had puppies.