The 75% rule makes some sense. However, "small market teams" sell their players because they cannot afford long contracts. It's the raw salary; they seem not to care about the luxury tax.
Don't forget, though that teams colluded for years to avoid each other's free agents. A "gentleman's agreement" that held down salaries, which is background for the current system. Most of the time, though, teams own their best players until those players are nearly 30, and "the industry" now says to ignore 30-year-olds: "don't pay for what the guy was".