Pin collecting has been a popular hobby for quite a few years because it's an affordable, portable souvenier. They are usually quite distinctive to the area they're from. When I was a kid, I used to get pins from every place we visited on vacations, I have 100's around somewhere.
Pin trading has been a big thing among Olympic visitors for a long time. When the Winter Games were here in Calgary in 1988, there was a large area set up just for people to go and trade and it was one of the busiest places in the entire city. I believe it's like that during most Olympics. At the Olympics, one of the things that happens is the major sponsors will produce limited edition pins that they only give to their employees or customers, so the only way to get some of these pins is to trade with someone who has one.
Somewhere along the way, Disney got into the business. I think they've sold collector pins for a long time (I believe I have one from my first trip to
Disneyland in 1987), but they recently began pushing pin trading as an activity for visitors to participate in at the parks. From the guest's perspective, it is a good way to meet new people at the parks and strike up conversations with cast members. From Disney's perspective, it's a great way to bring in more money.
To encourage participation, Disney provides lanyards full of pins to their cast members. The way I heard it described by someone at Disneyland was that these pins are "Mickey's pins" but because Mickey is too busy to be trading, he gives them to his cast members to trade with the guests. The cast member pins can be very nice, and often include "lanyard pins", which are only available by trading with a CM.
The rule is that any CM who is wearing a lanyard must trade with any guest who wishes to trade. Every pin on the CM's lanyard is available for trade one-for-one, and the CM can't refuse a trade as long as the offered pin is a metal pin with "© Disney" stamped on the back (because they are "Mickey's pins", you're not actually forcing the CM to part with his own property so don't feel bad if you get a much better pin than you give up - that's part of the fun).
There are also areas set up in the park for Pin Trading with other guests...They don't have to follow the "1-for-1, no refusals" rule, so if you're worried about getting taken by a shark, stick with CM trades only.
Disney Stores used to sell pins (often at deep discounts), but they seem to have stopped selling them (probably because the deep discounts meant low profits). If you can find good deals, eBay might be a good source of cheap pins, which you can then trade for nicer pins when you get to the parks. Just remember, you can only trade metal pins with "© Disney" stamped on the back.
I went to DL last year and had read about pin trading but didn't plan on doing any. Then, I went to Who Wants to be a Millionaire, somehow got into the Hot Seat and ended up winning 10 pins. The WWTBAM pins are all the same with the different point levels on the bottom - so I traded all of mine except for the highest level one (32,000) with different CM's in DL and DCA. I found that there seemed to be more CM's with lanyards (and better pins on them) at DCA then at DL - not sure why, and I'm not sure where the best places to trade at WDW are, since I haven't been there yet. At DCA, I found the best place was in stores along the back in Paradise Pier - almost every CM had a lanyard, with nice pins on each one (and there weren't many people in the stores, so the CM's were more than willing to talk and trade). I got a nice Stitch pin with flashing lights on it from one of the CM's in there.
If pins aren't the kind of souvenier you can see yourself wanting to collect, I wouldn't go out of my way, or spend a lot of money, to pin trade.
However, it is a good way to spend your time and meet some new people. When I got home, I gave the pins to my nieces and nephews, so it can be a good way to get souveniers for people back home if you don't want to keep the pins yourself.
If you decide to do it, do it for the fun of it. Don't go into it thinking you're going to make a fortune selling a rare pin on eBay. Try to trade up - lower value pin for higher value pin - but also look for pins that you like or think someone you can give it to will like.
