Is there a restriction on iPhone purchase?

Yes. Apple has recently announced this. Here on, you may only purchase iPhones on credit card (No cash). Also, you may purchase not more than two units. So, Why is Apple restricting the sales? Because, of the 1.4 million units of iPhones sold in the fourth quarter, 250000 were either resold or went to the unlockers for the phone to be unlocked. So Apple wants the iPhones to only go into the hands of genuine buyers.