Its an interesting question because if the Celtics do trade him, and this has already been brought up, I hope its for someone who can fill a desperate need, like a wing scorer or a dominant big man. DMC comes to mind. Its a pipe dream I know that, and the Kings are going to ask for the farm (not to mention they haven't given up yet on their season), but Sully, Lee, Hunter, the Brooklyn 1st this year, Celtics 1st this year and other filler is a decent package for him depending on how bad Brooklyn is and how much Hunter and Sully can prove in the meantime.
Thats just one idea but the trade Sullinger proposal while his value is high may not be likable, but it may be the best course of action.