Yes, the animals are already dead when we buy meat from a shop. However, that is because we effectively hire someone to kill them for us.
The number of animals raised and slaughtered depends directly on consumer demand for meat. Each time we refrain from buying meat at a shop, we reduce consumer demand and ensure that fewer animals will be raised and subjected to suffering.
So although the animals whose meat we eat are already dead when we buy it, our actions directly increase the number of animals who will have to suffer and die in the future.