Abbagail,
Thanks for asking your question to planDisney!
As it happens, I have a teenager that is a huge pin trading fan. My Abigail (different spelling, same love for trading pins!) has a favorite spot in EPCOT. When you are entering the park, it is to your right. As you head under Spaceship Earth you will see the Pin Traders - Camera Center store. In front of the store, they will periodically set up a pin-trading station. Guests can view an array of pins on a board, and when they find a new favorite, they can negotiate a trade with their current set of collectibles. Let's just say that the deals are always very fair!
As for the Magic Kingdom, they do much the same at several of the locations where pins are sold. Our favorite is at the Frontier Trading Post in Frontierland.
BONUS TIP #1
If you search for "Pin Trading" in your My Disney Experience App, it will recommend locations around the parks and elsewhere to continue your search for that perfect addition to your collection!
BONUS TIP #2
Pin trading with Cast Members is not only allowed, it is encouraged. I love how they interact with guests, making the day in the parks more personal and memorable.
Please feel free to write me back directly if you have any follow-up questions.
Have a Magical Vacation!
Doug