If the Celtics wanted to trade Rondo this season, they are very unlucky.
First off, the position of PG has been the position in the NBA that has strengthen the most. Multiple high draft pick PGs are working out. No, all might not be stars, but most are high level starters.
Now, the emerging of PGs else where. Rubio come in and looks great. Lin in NY takes off. And now, Isiah Thomas has a great stretch becoming a starter.
Trading a good PG (which Rondo is, not a great one) is not a good position to be in. Fewer teams need one. Teams are finding that with the rules in the NBA as they are, it is becoming easier to find good PGs.
Therefor, the price to get a good PG has dropped. It makes this probably a bad time to try and trade Rondo in hopes of a good return.
Now, if a great deal comes up, Celtics pull the trigger. But more likely, it means being really patient and being open to finding other ways to add a star and other good players to go with Rondo in the future.