As far as I know, if someone offers to trade you can trade anyone you want to, but some you just want to hang on to. Like for instance I have a segway around the world pin that you earn by doing the tour. Then if you get chosen to go up on stage at the AC for the radio show you get a AC pin, which a friend of my wife's got when I managed to get her picked for it. We almost had a heart attack laughing so hard.
I just want to add that ebay is a good place to get some pins. I was able to get some retired pins that completed Lil'Mermaid's collection and some old Minnie pins for Princess #2.
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