Like the Oracle of Omaha, Roy Carroll has made a fortune buying low and rarely selling. His sweet spot: cheap land that he’s turned into lucrative apartment complexes across the Southeast. Now Greensboro’s richest resident is biding his time, waiting for the coming real estate market collapse. At a monthly outdoor barbecue in a parking lot in downtown Greensboro, North Carolina, roughly 80 employees of the Carroll Companies gather around a plastic table piled with trays of hush puppies, mac and cheese and gallon jugs of sweet tea. Clad in khakis, a striped polo and an Apple watch, Roy E. Carroll II blends right in, loading a paper plate with…