dburg, exactly! This is how I am, too. I just love the chance to interact with other pin folks, and to exchange some smiles.
The only ones I pay attention to "rarity" is the Hidden Mickey pins on the cast lanyards. I rabidly search for them on the cast lanyards and try to complete my favorite sets during my trip. But I don't usually put those on my lanyard, so there is no confusion about trading them. The exception is if I have spares, which I will display for trade and will try to trade for other Hidden Mickey pins with fellow guests when given an opportunity, so we can help each other complete those sets. I will trade a repeat/spare for another spare if it will help someone finish their set!
On my last trip, I was there during a pin trading weekend--kind of a convention-like thing at Epcot. There were tons of people walking around just displaying huge lanyards (sometimes multiple huge, heavily laden lanyards), and they wouldn't trade. They were just showing off their collections. That was confusing to me, I guess.
There was one gal I ran into on the Jungle Cruise on our last day, hours before we were to leave for the airport. She had the ONE PIN, I kid you not, that I had been searching for my entire trip, to finish off my little triad of Hidden Mickeys. I asked her if she would be willing to trade, and she held out her lanyard.
"That's the one I have been searching for this whole trip!" I said excitedly and pointed to the one I needed. "I leave in a few hours and I never thought I would find it!"
She looked over my pins and said, "You don't have anything I want."
"Oh, I understand if that's your only one and want to keep the set together," I said dejectedly.
"Oh no," she said blandly. "I have a few, but you don't have anything I want."
I was dumbfounded. She had multiples of the one I clearly was thrilled to finally find, and I was leaving that very day...and she wouldn't trade because I didn't have anything she wanted to build up her collection? This to me is the antithesis of what pin trading is supposed to be about! And it jaded me toward trading with guests in a big way.