My oldest (now 9), got into pin trading about 2 years ago, and absolutely loves it. My little one (6), decided to start last year's trip, but was more interested in buying, what she likes, but started trading a bit more the last part of our trip.
I have bought about 30 pins a piece for the girls on Ebay, for about 2.75-3.00 each, which is a huge savings from the prices at the parks. I'm sure they will still buy ones to keep, from the resort, and special ones.
Last trip, I'm sure my daughter traded some more valuable pins for lesser ones. At $3.00 per pin from E-bay, I don't mind them just trading for what they like.
At home, they enjoy displaying their lanyards. We have them hanging from shelves in their rooms. They were also nice for show-and-tell at school.
I would suggest the locking backs for the kids though. They are a bit more, but worth it.