I agree with the above poster.. When we started collecting for my dd - we bought her pins that meant something to her.. (her first plane flight, first time on Splash mt., fav. princess.. etc).
Now , for the last trip, i bought her some pins (10) on ebay, and we solely traded those (it was actually a lot of CM pins!) - Honestly neither she or I care if they are CM only pins, we just collect what we like. That's the fun of it.. We are not in it for the "value" of the pins. And we only bring her pins to WDW that she can trade, since the black mickey backs on the pins STINK... she leaves her good pins at home on her pin bulliten board. This way - she knows anything on her lanyard, she can trade. It's much easier that way.
Now, i have to admit, i myself, just bought 2 pins on our last trip in Nov.. LOL, so i am slowly deciding how to get into this myself..
It can become an addiction, and can be overwhelming if you try to "do it all". So, like the above poster said, I am thinking i will only be collecting Tink pins - she is my fav. But, i haven't FULLY decided on that issue yet! LOL
Good luck and have fun!!