The dog may LOOK healthy, but can you see his bones? His potential eye issues? His heart and other organs? no.
Puppy mills are breeding solely for profit, and as such, care very little about the health and welfare of their dogs, as vet treatment and adequate housing and care would cut into their profits.
B*tches and puppies are crammed into cages - often so small that they can't stand up fully or turn around. These cages are so crammed that many dogs and puppies die in the fights that break out due to the close proximity the dogs are forced to live in (almost standing on top of one another). Their feet and legs can also be lost when they get trapped in the gaps between the bars of the cages and are torn off. The mill is unlikely to waste money on treating this, so will often either kill the dog (not necessarily humanely) or just let it bleed to death.
Very often, there is no heating or air conditioning, which leads to many dogs freezing to death in inter, and many more cooking to death in the metal cages which are effectively ovens in the summer. The psychological effects on the dogs are also awful. These dogs have never been allowed to move, in effect, and develop some awful mental and psychological issues.
They breed from their b*tches at every heat (including the first, when her body is not even close to ready to bear puppies), for as long as she holds out. They literally breed her to death. When she cannot reproduce anymore, and is a quivering incontinent heap, they kill her. She may be only 5 or so years old in many cases.
A common problem with dogs rescued from mills is that their jaws have rotted away after tooth decay was not prevented or treated. They also frequently have untreated eye ulcers.
The mills have jumped right onto this "teacup" trend and so are breeding very unhealthy dogs, usually runts, together to produce the smallest puppies possible. To aid in this, they may induce the puppies early, meaning that most won't survive. The have also been known to starve the pups during critical growth periods to keep them small (and ill). On top of this, the puppies may be taken as young as 3 weeks old, and sold as a "teacup" dog.