So why doesn't Google pay the supermarket to access that purchase data? Then it could, say, show me toothpaste ads when it knows I'm due to buy more toothpaste.
Next, are there brands using Google to advertise toothpaste? Do they need that data to do that? Are there possibly other signals that are as telling / effective?
Finally, how - without being more creepy - is Google going to buy that data and tie it back to you *exactly*?