Joe Thornton was traded in November and won the Hart (NHL MVP) in the same season. He was already an established star but he was traded in a panic move to try to save the GM’s job (needless to say that failed)
Terry Sawchuk, one of the all time great hockey goalies, got traded to Boston in His prime, then traded back to the Wings a couple years later. He won a couple Vezinas around those years.
Mark McGwire traded by the A’s to the Cardinals in 1997, although the A’s got more out of that than they expected as McGwire was on his way out of Oakland (and into St. Louis) at the end of the season anyway, but IIRC the Cardinals were in the pennant chase and thought they needed him to finish the 1997 season.
Wilt Chamberlain was traded twice: from San Francisco to Philadelphia in 1965 (Warriors had financial problems) and in 1968 to Los Angeles.
Hockey had a big trade of several stars in November 1975. Boston sent Phil Esposito and Carol Vadnais to their bitter rivals New York Rangers for Jean Ratelle and Brad Park. Both teams trying to revitalize once good teams in a slump. Boston had more success.
1926 in baseball Cardinals traded Rogers Hornsby to the Giants for Frankie Frisch. Hornsby, who was also the manager, was not easy to get along with and criticized ownership for playing so many exhibition games during the season. Giants manager John McGraw got nastier as he got older (he also had nagging health problems) and used his best player Frisch as public whipping boy. Frisch played great in St Louis for a decade. Hornsby got traded to the Braves the next year after publicly supporting center fielder Ed Roush in salary negotiations (Roush was a tough negotiator and also didn’t like spring training or John McGraw). A year after that, the Braves traded Hornsby to the Cubs for financial reasons.
My fucking Dodgers let Pedro Martinez go “we don’t think he has the build to be durable in the big leagues”. In our long and storied history of bad personnel moves, that has to be among the very worst. We got an (apparently injured) Delino Deshields. Pedro went on to win 3 Cy Young awards.
In late 1980, the Cardinals traded Rollie Fingers (relief pitcher, who had already largely established his Hall of Fame credentials, and whom they had just obtained from the Padres), Pete Vuckovich (not of the same caliber, but a solid starting pitcher at that point in his career), and Ted Simmons (six-time All-Star catcher) to the Brewers, for Sixto Lezcano (a solid outfielder), Lary Sorensen (solid starting pitcher), and two prospects (David Green and Dave LaPoint).
Fingers, Vuckovich, and Simmons all became key pieces of the Brewers team of the early 80s; Fingers won both the Cy Young and MVP in '81, and Vuckovich won the Cy Young in '82. None of the four players whom the Cardinals received performed at anything even close to those levels, and the two veterans (Lezcano and Sorensen) only lasted one season in St. Louis (though Sorensen wound up being packaged in a trade that brought Lonnie Smith to the Cardinals).