A Marcus Smart trade would have to be a dumb trade.
It either be dumb of the Celtics to trade him for anything less than a king's ransom, and it'd be dumb for anyone to offer a king's ransom for Marcus Smart.
He means more to the Celtics than he does to anyone else, he's in his first year of a team friendly contract, he is a freeking "Velociraptor on defense" (quote from Zach Lowe), he is invested in the vision of Brad Stevens, and his offensive game has made leaps and bounds halfway through his first season.
It would be dumb to trade Smart for anything less than established elite talent.
and
It would be dumb to trade established elite talent for Smart's potential.