If beauty is in the eye of the beholder, a great fantasy trade is in the eye of the owner pulling the trigger.
Ultimately, a player's value in fantasy will come down to a number of factors. Some of them should be objective. Is that player producing? Does he have a history of producing? Is he injury prone, stuck in a position platoon or inconsistent?
But some factors are subjective. We've all had players who have bombed for us before turning things around the next year. That won't change the fact that you'll always be wary of that player now.