I'd be curious to know how many people on PinPics might be casual collectors or kids. By 'casual' I mean that they really have no concept of the rarity or value of the pins, and just like to collect things they think look neat.
Case in point- my 4 lovely nieces that got me into this hobby... At the time, all pre-teens. They get excited at the sight of any cast member at the parks with a lanyard, and are always buying the common pins at various shops, trading, etc. They all have accounts on PinPics (at least 2 of them do anyway), and they try to trade occasionally. Only being exposed to the park trading/buying environment, they think the hidden mickey and cast lanyard pins are the rarest things out there.
So they'd see something interesting on PinPics, and offer a hidden mickey pin in exchange... Not really realizing that they are trying to trade for an LE25 auction pin or something... They'd never dream a pin could cost more than $20.
I buy them pins from DSF occasionally, and they've offered pins that became quite collectible (LE150s or something) for a much lesser pin (but one they wanted). Of course the other collector traded, he/she was happy to get a rare pin, my niece was happy to get a pin she really loves, no harm done... They just think it's fun to trade pins... I've even given them rare-ish DSF pins that they didn't care about and told them to offer them on PinPics for things. They'd probably offer them for a common non-LE pin, but they are guaranteed a higher 'Yes' rate, so I'm okay with that.
Of course, this isn't the case most of the time probably, especially with people that have 1000s of pins and make a living on eBay, but there's always the chance that some of them are just people that don't really know there's a rarity involved with these things...