A topic dear to my heart. My three kids started pin trading in 2001, I still have their "pin trader badges with their pictures" that they made in Ca Adventure at the photo booth. Probably can't do that anymore.
The pins are indeed owned by Disney and cast members are just trading them for the Mouse. If anyone that is an employee of the park, whether working at a ride or wearing a business suit and walking along the street, is wearing pins they are willing to trade.
The employees either wear them around their necks or on a fanny pack that looks like a square piece of fabric hanging around their waist. You walk up and say, "Would you like to trade pins?" or "Can I see your pins" and they will show you them. You can ask for up to 2 of their pins and you must trade one pin for one pin. The back of your pin must have "disney" on it. You can either be wearing your pins or have them in a pocket or backpack. My little boy who is now 15 refuses to wear a lanyard but still loves trading pins, so he carries them in his pocket.
Some lanyards, they used to be teal blue, are only for children. If an employee is wearing a teal blue lanyard, he or she will only trade with kids. They will tell you if you ask them by mistake. No big deal.
My kids love to look for the Cast Lanyard pins. They have little hidden mickeys on them and you can only find them by trading with a cast member (but I think they are changing this rule and you will be able to buy lanyard pins, but I'm not totally sure). And my kids like to check the back of the pin after they trade to see if it is a limited edition (only a limited number of them were made) because they think it is special. Pins usually come in a series, so there will be a set of them that are similiar. For instance there is a set of pins that look like the
Disneyland trash cans, each pin resembles a trash can from a section of the park.
Cast members don't have to trade, they only wear pins if they like trading with guests, so it is usually a great experience. And if you are looking for a pin to complete a series, mention it. I've had cast members reach into a pocket and pull out the missing pin that we've been trying to find.
Have fun!!