The 7:30 pin trading "event" at least on our cruise was a CM at a little pin display that had
DCL pins you could buy and then a book with ones you could trade out of or with her on the lanyard. Check this book out earlier in the cruise. The CM told us that by the end it fills up with the Castaway Club giveway pins because you can leave duplicates in the book unlike if you trade with a CM with a lanyard. If there are fellow cruisers down there you can always trade with each other.
As far as other CMs trading the ones that always had lanyards on were the gift shop employees and the CMs handing out hand sanitizing wipes at the entrances to the restaurants. My kids had the best experience trading with these CMs. Occasionally the people at Guest Services will have their lanyards on. You get accostomed to spotting those lanyards because sometimes the CMs with them on is random and you'll just bump into one.
There was one night of "crew trading" as was listed in the Navigator (it was the last night on the 4 day). We were all excited thinking the captain or 1st officer may be there, but itwasn't but there were 3 of the "crew" but they didn't really have anything spectacular on their lanyards. Maybe others have had a luckier time with this than we did.
Good luck! I can't believe how addictive it is....