The receipt has the last four digits on the card. Localizing that specific card number to the store and you are going to get pretty complete profiles on people. There will obviously be some duplicates but some smart algorithms could match up the common purchases. If I always buy the same boxed tea and the same deli meat, patterns can arise. The guy who buys a 1/2lb of swiss and 1/2lb of roast beef every week is probably a different person than the one that buys 1lb of cheddar and 1lb of turkey each week. Associate my other purchases with my deli meat choices and they can track when I came in for some special dinner ingredients I've never bought and a box of my usual tea bags. Even purchases that look unremarkable at first glance, like bread, eggs, and milk, have trends (bread brand, egg size/brand, 1%/2%/whole/skim/brand of milk). Having the last four digits makes it a lot easier to track customers but even without that, there are probably unique customer trends you can pull out of the seemingly anonymous data. At minimum the store could print out some coupons that it thinks you would be interested based on your latest purchase and what other people buy with the things you buy. A mom buying capri-suns and lunch snacks would probably be interested in similar kid lunch items.