When every transaction is trackable, everything is taxable and able to be fed into big data.
"Sorry, sir, we can't sell you health insurance. Our records show that you've been purchasing too much alcohol."
"Excuse me sir, we're from the local police. We're concerned that you've recently bought 500 rounds of 22lr ammunition (and that's a perfectly reasonable amount). Mind coming down to the station and answering some questions?"
Edit:
And of course, the ever popular.
"Hey, we noticed you bought a used car. Based on your finances you shouldn't have been able to afford it, according to our calculations, unless you've been getting paid without notifying us. We already know what you spend and how you spend it, and how much money you make. You're going to prison for tax fraud."