Google's algorithm magically knows I was looking at shoes and advertise shoes at me. There is a minor flaw to their plan. I see the shoe adverts after having just bought the shoes. Now, do I want another pair. Not for a long time, but seeing the ads for the shop I just visited and did not buy things from is one thing but seeing ads for what I just sought out and bought is something else.
Google has better think this one over. It could put people off.