What should I look out for when buying a puppy

[neolihia]

Hi,
I am new to this forum and would like to tap on everyone experience here.
I've been wanting to keep a dog for years and have been doing lots of reading up about the breeds of dogs in terms of caring for them etc.

However, I've been unable to find information on where to buy dogs..or rather, are there any particular places I should avoid or things to look out for when I am buying a pup.

I recently went to Pets Lover Center and saw a Shih Tzu and love it alot. It's at $1380. I try to search for more pet shops online and came across ads of people selling Shih Tzu as well. Some goes for $900 and some $650, others are in this range as well.
I am just wondering, what makes a pup difference between a $1380 pup and a $650 pup.

I have read about puppy mills, home breeder..etc. I am not sure what should I look out for or verify. E.g. if I go and view the puppy, my criteria is definitely my "instinct" with the pup. But how do i find out if the pup have sickly parents or has been ill-treated. Or how old exactly are they. I read some forum that even pet farms fake the age of the dogs.

So, is it a general rule of thumb to say that it's always safer to buy it from pet shops?