(someone) could sell an icebox to an Eskimo

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