The idea behind trading Avery Bradley is that he has 1 year left on his contract, and will then need to be paid. In today's NBA, where Evan Turner gets 5 years and $75 million, Bradley will cost that, or likely more. Teams have to decide if spending $20 million or so on a very good role player is good business for the roster and teams future. For the C's, I don't think it is. In the NBA you win with stars, and you can't overpay good but not great players. The C's already did that to sign Horford, and they can't do it with Bradley imo. Especially if they look to sign a max FA (or bring one in), with IT needing a new deal too. They can't sign them all, and with there being a glutony of guards on the roster, with Fultz likely to come, one of them (or more) is likely to go. I think Smart is much cheaper to resign, Rozier won't net them much, so Bradley is probably the guy who goes. While he's a good player with strong aspects to his game, he does have limited skills and may have peaked as a player. If they don't trade him now, they run the risk of losing him for nothing next offseason.
Crowder is the guy that won't likely get traded, if for his tremendous contract alone. He is signed cheap for 3 more years I think, and is a starting quality player. I personally don't think he's going anywhere, unless the deal is too good to pass up.