sell snow to the Eskimos

sell snow to the Eskimos  {v. phr.}

To sell something to people who already have a large quantity of the same or similar goods.

My Alaskan friend said, "One of the hottest businesses in Alaska is refrigeration. You could say that I, as a refrigerator expert, am selling snow to the Eskimos."

See: CARRY COALS TO NEWCASTLE.