hockeybabe is right, but there are some things to watch out for. Some pins are of lesser quality-- they're made in China or something and even though they say "Disney" on the back they aren't official Disney pins and CM's aren't supposed to trade for them. Some people have been embarrassed about having those particular pins.
Another thing to look out for is lots of the same pin, for example, the Millennium pin. Disney knows that there was a surplus of those pins and that they're being sold on ebay for cheap cheap cheap. Ditto the Kodak pin. So, CM's deliberately keep one of each of those pins on their lanyards, prohibiting them from accepting another one.
You can find more details on the pin trading board. Good luck!