A few more things to mention:
If you visit the PinPics website, click on the link that says "PinPics U". From there you will see sections called "Getting Started", "Pin Trading", and "Building a Collection". They have a lot of good information about how to trade in the parks and what is allowed.
Also, PinPics has a really great search feature at the top of the home page. For example, if you type the word "balloon" into the search bar, you will see every single Disney pin that features a balloon. If you then click on one of the pictures, they will show you info about that pin, including the year it was released, if it was part of a series of pins, if there are known counterfeit pins out there, what the counterfeits look like. It's really fun to browse around to figure out what types of pins are the most appealing to you and if you'd like to start collecting around a certain theme or character.
I would also add that I think the best place to trade pins is in Epcot in World Showcase. My kids loved talking to the CMs from other countries! It's great because with a simple, "Can I please look at your pins?" the door will often open to a wonderful and educational conversation.