If you buy a puppy from a pet shop, you are not only supporting the cruel puppy mill industry, but you also run the risk of taking home a sick animal. In fact, the ASPCA, through its No Pet Store Puppies campaign, advises people not to purchase puppies, or even pet supplies, in pet stores that sell puppies. Respiratory infections including pneumonia, as well as hereditary defects like hip dysplasia and severe allergies, are common among the indiscriminately bred puppies from puppy mills.
If you have purchased a pet store puppy who turned out to be sick, you have may have some recourse—as of January 2010, 20 states have enacted laws, commonly called "lemon laws," that make pet stores financially responsible for sick animals purchased from them. See a state-by-state list of lemon laws and know your rights.