What a good question.
My only answer is you are taking a chance ....I saw that someone did get a puppy that is healthy - but in reality - most puppy mill puppies are NOT....you can read on the net for much more information but unless you find a very good breeder - you will take a chance that you may have health problems in the future....
Most mills and Pet Stores provide MINIMAL vet care and in some cases NONE. At many mills - puppies are infested with parasites or worse...they get sold to Pet Stores....many times the puppies are sick & die on the way to the Pet Store - the ones that make it...who ALSO may be sick...get Minimal or NO vet care there also. It's HORRIBLE and far more common than people realize.
It hurts to see puppies in bad conditions and we WANT to rescue them - but it hurts them ALL in the long run....when one is purchased - they have to replace it and the cycle continues....
The torture and abuse STARTS at the Puppy Mill...make no mistake about that....for the few and far between Semi-HUMANE Mills - there are many many horrible evil torture chamber type mills in this country. The bad ones are way too prevalant in this country so in order to stop this practice - they ALL NEED TO BE SHUT DOWN