I love pin trading
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I started in 2006 I think, having resisted because I just never had enough spending money to really justify starting. But we have about 4 lanyards full of pins traded from specific parks.
What you get out of it depends on the person, serious collectors might choose to focus on certain sets or characters. Others collect pins based on what they like design-wise and the memories those pins generate.
Personally I collect Pins because I always enjoy trading in the parks, it's fun peeking at CM's pin lanyards while waiting and finding a gem here and there, plus CM's always have special "Hidden Mickey" or "Cast Member Exclusive" Pins that can only be officially gained from a CM trade (or unofficially, ebay)
The best way to gain in trading is to buy pins online, get a bunch of cheap ones based on something Disney that your not that keen on, so you don't feel too bad about trading, and stock those up for the next trip. For example; last week I got a set of Club Penguin Puffles, because I'll be more than happy to trade those : ). Don't buy pins at Disney unless you REALLY like them, they cost a mint in the parks and much as I love them, I have to pick and choose what I want
On the flip side, there are pins that simply mean too much to me to EVER trade - the Wall-E set my DBF brought me, the special Year of a Million Dreams pins from WDW ect.