I agree, every fish purchased from a store is a rescued fish. You are saving it from a death right there in the store or from someone who doesn't care how they keep the fish.
My first rescue, a free fish, died in 2 days. He was really sick but I wanted to try. My second rescue I paid for. His only problem was half his anal fin was gone. I just keep him in water with stresscoat and it is growing back. I don't think anyone else would have bought him because of that fin. I did try to get him for free but the wrong manager was on duty that night