You shouldn't buy puppies from pet shops - they have most likely come from puppy farms. Reputable breeders will care about the homes their puppies go to, they won't send them off to be sold willy nilly.
So in answer to the question, that puppy in the window could turn out to be a nightmare that will cost you more than you can imagine. A lady I used to work with had a dog she'd bought from a pet shop, and one day the dog flipped, attacked her and her 2 year old daughter and ended up being put to sleep.
Sorry to bring down a jolly thread!!