Maybe I am wrong, but it seems to me that any player will lose market value if he changes teams every season and acts like Simmons did earlier or like Harden this offseason. Who knows, maybe Harden will purposely misbehave both on and off the court during the upcoming season, and the 76ers will have no choice but to trade him. Of course, it's unlikely that will happen, though I wouldn't be surprised by it.
Simmons and Harden are actually two quite great players but some people may not really like them but it seems like these two players experienced decline in market value after officially moving or swapping teams last season.
It just that when has selling price low on the market it possible that the team will never sell him.