Paper tickets aren't out, but the Oakland Athletics believe leading off a Major League Baseball mobile ticket-buying option will help sell more Coliseum tickets to more fans. Online ticket buying already is the hottest channel for the A's, said Steve Fanelli, executive director of ticket sales and operations for the A's, and fans have been able to use MLB's Ballpark app to check in at games or upgrade seats when already at a game. Join the conversation: Follow @SFBusinessTimes on Twitter, "Like"…