Sorry, but I don't think anyone bought rifles and carbines during combat operations on Okinawa or anywhere else. In the post war period, after surplus guns became available to arms dealers, they did show up in post/base exchanges and military personnel bought them there. And military personnel who were NRA members could buy guns through the DCM just like any other NRA member, as military personnel can buy from the CMP today. But that is not the same as simply tucking your issue weapon in your bag and bringing it home. That was and is illegal.
And I did hear about one officer who "sold" rifles, carbines, pistols and SMGs to his men. He simply claimed the guns a lost, and pocketed the money. He may still be in Leavenworth for all I know.