I like to have a few of my least treasured pins to trade just for fun! It's a free activity and if you see a pin board with something you like, you get to trade out something you don't like as much!
Kids in lines are fun to trade with, too! I did notice that the CM'S had a lot of duplicate pins throughout the parks. The best, most populated board I saw was in the information booth outside the security area!
For serious traders, I have no idea if anyone is still doing it with collectable or valuable pins... I know there was a guy with a sprawl of those Vinylmations in DTD.