Puppy mills and reputable breeders are not the same thing. Reputable breeders usually have a reputation that is known amongst other breeders and dog owners. They often belong to organizations that certifies them or recommend them. If you are looking for a breeder who is legitimate and reputable, you can usually research them and find information about their litters. People will tell you about them. Their dogs are usually of healthy and good examples of their breed. A reputable breeder cares about the animals they breed. I would not refer to a reputable breeder as a puppy mill.
Puppy Mills are designed for one thing only to sell dogs to make a profit. They hid from the law and work in the shadows. The conditions are deplorable. The dogs that come from their facilities usually have genetic diseases, chronic illness, or emotional problems. They do not breed for health of the animals. They breed for the money they can make. If the female dies in the process, they don't care. If the puppies are sickly, they hide it . They only need the maximum amount of litters born so they can turn a profit.
I would never equate puppy mills driven by profit to breeders who sell dogs but do it in a caring way. The two should not even be compared.