Except we know that tracking someone by their purchases is trivial. After all, do you plan on having a cell phone? Then, any card used to pay that phone bill will indicate who all of your connections are.
If you just purchase everyday items, you better be randomly wandering the entire city to make those purchases. Otherwise, they will have clusters of locations to have an idea of where you live. Of course, you will have to walk everywhere since taking transit will also use the card.
Now, you are safe IF you can pull of anonymity - but that is a far, far harder task than it seems.