You hand the cashier a credit card, she thinks you're cute, too. Cashier reads name on credit card, wonders "if he's on Facebook", looks you up and there's your connection. Something something Occam's Razor.
I was about to ask why Bluetooth would come into play, but then realized that I replied to the wrong comment (the intent was to reply to the one about the "super cute cashier"). Though giving some thought, your way would work, too. Assuming that one can get the MAC address on a mobile device (never tried on either of the major platforms).