A puppy farm is a breeder who produces large quantities of puppies every year. With little care for the animals health or welfare. Puppies produced this way are far more likely to have health or temperament issues.
From living in a bad environment
One off breeders. Will have max one advert a year.
Have a few dogs that they breed from following animal welfare regulations. Will have less than 4 adverts a year and less than 2 breeds.
Have many dogs for breeding and will little care for animal welfare. Will have more than 4 adverts a year and multiple breeds.
If an advertiser is posting lots of adverts it is likely to be a puppy farm. If the puppies are over priced this is another bad sign. All advertisers posting more than adverts four adverts a year are likely to be a puppy farm.
If an advertiser is selling multiple breeds they are likely to be a puppy farm.
Choosing a dog from an ethical breeder stops the profitability of unethical puppy farming.