At breakfast and lunch, the CMs at the entrance to cabanas and the sit down restaurant wear pin lanyards to trade. As mentioned previously, there is also officer pin trading, pin books in the shops, and pin boards on stateroom doors.
As far as wearing pins, my daughter has two lanyards. She wears one with pins when she knows she is trading, and has a blank one to wear when she's going to the clubs oe just generally moving around the ship.