Me and Haki are having a bad week, first I see "Haki is Spider Sense" and now this... Really, there's no reason to assume that Buggy has a Haki for several reasons. Firstly, the usage of Haki for specific combat uses is something that only the Kuja can do. Secondly, there's no reason to assume that the Muggy Ball is anything but an extremely small version of the Buggy Balls seen in the Buggy arc, and there's no way that those were Haki imbued at all. They were just cannonballs packed with explosives, the same thing here. And thirdly, Buggy has nowhere near the strength and force of presence to generate a significant Haki, certainly not enough of one to destroy walls. He's stronger than he was in volume 3, but not so strong that he can knock people out just by being near them, he's still pretty weak, as One Piece characters go.