Isn't this already happening? I got a tour from one of the techs at a major general store on how they track users "to enhance user experience". It's both impressive and terrifying. Using simple things like tracking the wireless signals on your smartphone they can see exactly when you pass or enter the store, which departments you visit and stay longest, etc. The system is even linked up to a camera at checkout so they have your picture linked to all other details they gathered.

In the country I live it's also very popular for every store to have their own plastic card you gather points on at every purchase (e.g. for discounts or freebies). This is an easy way of matching your purchases with all the other personal information they gather.