I think it's been one of those things where the basses with the emotional attachment to have been replaced by something else. By the time they have been sitting in unplayed for a year or so, objectively unless I want to own lots of basses, I may as well let them go someplace else.
Or in the case of a couple I've let go, the bass the cash allowed me to get was far more valuable than the emotional value of the bass to me.