We sailed a b2b2b on the Dream (May 1st, May 4th, and May 8th). There was pin trading several different times during all three cruises.
I was looking for a pin that would represent having sailed on all 4 Disney ships. The CM who was helping me, opened up a large trunk which was filled with pins. He stated that the trunk was usually only opened during pintrading. The truck was approximately 4 foot tall by 4 foot wide, and about 2 feet deep. The trunk was filled with pins.
The Navigator will list the specific times, but there are CM's throughout the ship with pin lanyards on who were trading whenever we saw them.