Interesting thread- I've never bought from a breeder actually, not that I'm against it, I just typically end up falling for bettas from LPSs. I maintain about 15 bettas at a time, I've lost some yes, but most I've had for years. Been to some horrible stores and some that are willing to learn...
I can see the point of not rescuing fish because that will perpetuate the problem.... Rescuing (aka buying) fish from the shelves does in a way create a vacuum per se, a place to bring in more sickly fish in need of rescue.
However, that is unfortunatley irrelavent, because sadly at the end of the week (or 2 or whatever) the petstores generally make room for more fish by flushing the dead or diseased fish down the toilet. They will make room on the shelves regardless.
So until they don't sell bettas at all, I will continue to rescue fish.
And sorry to say we cannot argue that buying the bettas is what keeps them selling them, because they do not actually make a profit from selling bettas. It's fish accessories that make profit.
ChoclateBetta I love your signature!