Answer
Don Cardwell was traded to the Mets in exchange for Tom Seaver in 1977. Cardwell was a veteran pitcher who had been with the Mets since 1975, while Seaver was a young star who had won the Cy Young Award in 1973 and 1975. The trade was a major coup for the Mets, as Seaver went on to become one of the greatest pitchers in baseball history.