… is from James M. Buchanan writing in 1980 (in his article entitled “Rent Seeking and Profit Seeking”); Buchanan is here quoted on page 17 of the first (1981) edition of Robert B. Ekelund, Jr. and Robert D. Tollison, Mercantilism as a Rent-Seeking Society:
But let us be honest. How much more do we know about market process than Adam Smith knew that is of practical relevance?
(The much-revised, 1997 edition of the Ekelund-Tollison study on mercantilism and rent-seeking is available here.)