"The only reason we sell is to fuel our ability to buy, " says David's son Helly, 29, who runs a gallery in the posh Carlyle Hotel on Madison Avenue. (Helly's cousin, also named Helly Nahmad after their grandfather Hillel in the Sephardic Jewish tradition, runs a gallery in London.) And while there is a cost to holding that inventory, the family's currency business helps support their art strategy.