Okay, probably a stupid question, but couldn't the Celtics have traded Zeller for Bogut and maybe thrown in a 2nd or something to make it worth their while?
Yes, they could have. Probably could have done it without a second-rounder. But Bogut has been hurt a ton this year, and is likely to be less useful than Zeller going forward. He'd be more useful than Jordan Mickey, so that's why you wait for buyouts.
Also, Zeller's contract has a chance to be a useful trade piece around the draft, because it's non-guaranteed for next year. But, in my opinion, it's more that you don't trade your only pure center (even if he's below average) for someone who's only played in 10 of the last 36 games due to injury, has a long history of not making it through seasons, and has been of only questionable effectivess even when on the court this year.