(someone) could sell ice to Eskimos

Someone is an extremely smooth, charming, or persuasive salesperson, such that he or she could sell something to people who have no need or use for it. (The phrase alludes to the various indigenous peoples of the traditionally cold, snowy northern circumpolar regions of the globe, who thus would have no shortage of (and no need to buy) ice. It should be used with caution, though, as the term "Eskimo" is considered by some to be offensive or derogatory. ) I can't believe you were able to sell an extra 200 units to the hospital. You could sell ice to Eskimos!