This thread has taken a timely turn.
With my rankings for my own small online store in the tank, I'm doing AdWords, although the ads don't do anything. What is bringing in buyers is Google Shopping/Products.
Google has changed the format of those listings in the last several months. Where before sites were jockeying to get ranked well in Products, Google now has a bunch of merchants under one listing, based upon price. Click on the button, and you get five or ten or twenty retailers for the product.
It almost looks as though Google is moving to become the middleman, and eliminate the need for retailers to have a website. All they need to have is the shopping cart and the product pages.
I know it sounds tinfoil hat, but it looks like Google just might want to take over ecommerce by making themselves the conduit to the shopping cart.