Brokers said many wealthy Cape Codders are trading up. Other buyers see the Cape as a strong investment, with bigger properties still selling at a 20% to 50% discount to the Hamptons in New York.

CNBC has learned that billionaire William Koch—the sailor, wine-collector and brother of famed conservatives David and Charles—is putting his historic compound on the private island of Oyster Harbors up for sale. Robert Paul Properties, an affiliate of Luxury Portfolio International, is about to list the property for $15 million.

Koch is selling after he purchased a nearby estate previously owned by Rachel "Bunny" Melon for $19.5 million, according to reports.

The top listing in Martha's Vineyard, the Obamas' favorite summer retreat, is a property called Herring Creek Farm, a 25-acre estate on the market for $19.5 million. The farm sits on a private corner of land off Slough Cove. Located on the southeast part of the island, the property features sweeping views of Edgartown Great Pond and the Atlantic, writes CNBC.