Well, it's easy from the data point of view. You basically have two data sources: (location, time, credit card) and (location, time, MAC). The location/timestamp history of any given individual is one of the most unique pieces of data you can have; correlating the two is a trivial data-science exercise.

It's the systems-integration part, moving the data from the card swipe onto the right computer, that's actually hard. You might be able to partner with your payment processor to get at that stuff.