At the moment his value is much more valuable to the Celtics than to anyone else.
First, the Celtics have him under a team friendly contract for at least 3 more years.
Second, he makes diddly squat so any established player you want to trade him for will be difficult because you will have to make the salaries work, which means sending off another higher priced player with him.
Third, this might not even be his breakout year as he is just 21 and still developing. The C's could be trading away the next Michael Cooper or Joe Dumars, you just can't tell right now. Heck, how many people were calling him garbage and other such stuff on this sight just 3 months ago. Look what happened in three months. Imagine what could happen in two years?
While I doubt he is untradeable, I do think the thought of trading Bradley right now is nowhere even close to being an option in Danny's mind and he would need to be bowled over by an offer to consider it.