NEW HAVEN, Conn.--As Zach Rotholz waited for customers in one of the 54 hours a week he spends at his New Haven, Conn., Chairigami store, a middle-aged man walked in.John Ziebell stopped in after seeing the cardboard furniture displayed in the York Street window, with lots of questions."This supports weight?"Rotholz, a tall young man with a ready smile, told him to sit down and try it out."Oh wow, it's sturdy. You guys sell this stuff?"In fact, Rotholz has sold between $6,000 and $7,000 worth of his cardboard designs in the six months since he launched his business just a few months after graduating from Yale with a mechanical engineering degree.