Usually it is ok to buy ticket for someone else. I do it with AMEX. Once when I did rebooking for return flight (SAW-VIE) for my friend it even stated that CC will be asked to see at the airport at check in and indeed it was. Must have been physical so my friend was denied and flew Pegasus. It was interesting because I paid for his return flight and there was not any note about CC for the whole itinerary, just for the change later on.