My thoughts: you wouldn't be rescuing those animals. You'd be contributing to the store's poor habits. Buying animals from them will only put money in their pockets and more animals in poor conditions. I know it sounds valiant to take those animals out of those conditions, but it would be like rewarding the store for treating the animals that way. They're there to make a profit. You buying the animals is only giving them what they don't deserve.
We can't save them all. But if people stop buying animals from there they'll stop trying to sell them.... eventually.