I wouldn't bother helping the pet store by buying her. Paying even a penny just shows them that they can keep the animals like that and someone will still want them.
It's not really rescuing her IMO if the store benefits from you getting her. Even if they gave her to you for free, you're still helping the store by freeing up space for another one. So, you removing her from the situation in any way is helping the store.
Sometimes, when you want the store to stop carrying certain animals, the best thing to do is leave them so the store learns that they cannot keep the animal correctly. They figure out that they end up loosing money on the animal because of the inadequate care.
Of course you should be trying to inform the employees first to see if they can bring up their care standards or stop carrying the animals all together. The best situation would be that you can get them to get better husbandry wise or stop stocking AND they give her to you for free. Ultimately, this is the situation you want.