The other thing about pet stores isn't just unhealthy rats, it's also poor genetics, and poor ethics. Most pet store rats are bred indiscriminately to be food. The mother bears litter after litter until she is too exhausted to do so anymore, and is then stuck in the feeder bin herself. Males and females are often allowed to intermingle, and young girls often come pregnant (as many members on this forum can attest to!). The often have underlying genetic conditions, because none of the rats lived long enough to know to breed those conditions out.
Most pet store keep their rats in substandard conditions, and don't care a whit for the welfare of the animal. And even if you think you're "rescuing" rats from being snake food, the truth is that you're still paying money to these people to continue their behavior, and in the grand scheme of things you haven't done much good at all. Of course, to that one rat you did save, it meant the world, but to the other rats that were bred because you told the pet store it was okay, it wasn't such a good idea.
If you think that your petstore is different, and feel that it's an ethically sound decision to support them by purchasing their rats, and they are healthy and well socialized, then go ahead.
And all that being said, though, two of my girls I bought from PetCo, the third was a rescue, originally from PetCo. I'm taking my chances, and in hindsight I'm not sure I wanted to support PetCo like that (though they're actually the best store in town, which really says something, so I'd be supporting them no matter what), but looking at my girls, it's hard to feel too bad.
