Definitely look at the guide on the Disney Pin Trading site the PP posted. Basically, it must be an "official" Disney pin. Any pin with the pin trading logo on the back should be ok, along with older character/park pins before they put the logo on the back. I say should be ok as there are fakes floating around and some CM's may refuse to trade if its a fake. Trading with CM's is a 1 for 1 trade, and they aren't supposed to say no as long as they don't already have the pin you are offering on their lanyard. Trading with other guests is up to you, you don't need to trade with anyone, but you are more than able to, especially around the pin store at DTD.