I think that the red line is definitely crossed whenever a third party is involved. It's okay for a shop to optimize what they sell and recommend things based

I think that the red line is definitely crossed whenever a third party is involved. It's okay for a shop to optimize what they sell and recommend things based on customer data. What is not okay is for a shop to share customer data with any other third party. Note that by third party I do not only mean advertisers, but I also mean hosting services such as Amazon, Google, Facebook etc.: if you think at them as long streets with shops on the sides, I do not think many people would appreciate having said street filled with surveillance cameras that track everybody's movement and purchases and sells this information.