If you trade with a CM, there are a few rules:
1) The pin must be a Disney pin (some operating participant pins are ok too.)
2) CMs are not permitted to have more than one of the same pin on their trading lanyards.
3) Only children (12 or younger) can trade with a CM who has a Green lanyard.
4) CMs are not required to trade their own personal pins, only those on a company-owned trading lanyard. They are not permitted to trade most recent year of service pins (small brass pins worn on the nametag).
Other than that, anything goes. One man only buys Grumpy pins and trades them for others. I met an older couple the other day who just recently got into trading. He is trying to collect nothing but Disney Dog pins, she is collecting Princess pins.
Disney's Pin Traders shop at Downtown Disney Marketplace has tables outside where many people gather to trade and talk about pins.
At each park there is a "Pin Central" location where this type of thing takes place (Innoventions Plaza at Epcot, Under the Hat at MGM, Exposition Hall at MK...) but you can trade whenever and wherever the urge strikes you.
Above all, HAVE FUN!!!