Final answer:
The assertion is true; dogs from puppy mills are often sold in pet stores, online, in classifieds, and through breeder directories, and it is a practice associated with numerous animal welfare issues.
Step-by-step explanation:
The statement that puppy mill dogs are sold in pet stores, on the internet, in classified ads, and in breeder directories is true. Puppy mills are commercial breeding facilities that prioritize profit over the well-being of the dogs. These facilities often produce a large number of puppies under poor living conditions. It's important for potential pet owners to be aware of this issue and consider adopting from shelters or purchasing pets from reputable breeders who follow ethical breeding standards.