Very nice and everything, but if I went into a store and the payment method was via a device attached to an iPhone then alarm bells ringing "card skimmer" would be going off in my head and I'd walk out the door.
Why? Your liability is zero, your potential inconvenience is low, and there's no reason people can't skim your card info using other readers too.
Why? Because I don't want the inconvenience. If I call up and complain about unrecognised charges then my card is cancelled, then I have to wait 5 business days minimum for a new card and also a new pin to be issued. If you want my business and my money then get a proper credit card processing terminal, there are plenty of other merchants around.