1973 - Astros trade Jimmy Wynn
At just 31 years old and after 11 years with the team, the Astros traded cornerstone outfielder Jimmy Wynn to the Los Angeles Dodgers in exchange for two pitchers. Wynn played just two seasons with the Dodgers before getting shipped off to his third team, but in those two seasons he played a key role for the Dodgers, helping them reach back-to-back 85+ win seasons.
Wynn is still high up in the leaders boards for all-time Astros offensively to this day, being ninth all-time in hits and doubles, and has his number 24 retired by the team in 2005.