But when you buy meat at the meat store, whatever "being" once lived in that meat already moved out of that house long ago. There is no karma from buying or eating this meat...
This is also Bhavaviveka's perspective.
The truth is that Christians, Moslems, Jews, and Secularists will never stop eating meat. They will never stop raising animals for food. If practitioners refuse to eat meat, they are refusing to create a good cause for the animal whose flesh they are eating.
There is no need to suppose we must therefore decide to eat every kind of dead creature and so on. We can work within the convention of what are considered food animals in our culture and society, i.e. poultry, beef, pork, lamb, goat, venison, wild game, fish and shellfish. It is also ok to enjoy the taste and the flavor of these kinds of foods. We have sense organs, we should enjoy what we eat. We should also be aware, we should not be blind to suffering. Also when we eat a salad, or a tomato, we have to be aware of the suffering the production of that tomato or lettuce, or head of broccoli engenders. When we pick a tomato, we are also picking someone else's food, the food of another creature. When we eat a strawberry, we are stealing it from some bird, chipmonk or insect. When we buy mass produced vegetables in a market, how many creatures died to produce that? When we use sesame oil to cook our vegetarian meal, how many millions of small creatures were crushed to death to extract that sesame oil? The idea that being a vegetarian is less harmful to sentient beings than being a meat eater is deluded. You can, for example, in the same cycle of treasure texts find one text that says you must avoid meat, and in another text from the same cycle, instructions that one must eat meat.
If you have a specific reason for being a vegetarian, for example, you are doing chulen (rasāyana) practice -- then you must avoid all foods that give rise to the three humors and focus only on sattvic foods, essence foods, such as ghee, honey, rice, fruits, etc. You cannot eat garlic, onion, radishes, etc., roots in general. This also has to do with how to cleanse the digestive pathways in the formation of the various tissues of the body. Even so, there are tantras that identify meat as rasāyana, chulen. So meat can even be used for chulen practice.
If one is a Dzogchen pracititioner, there are no rules about what one may eat.