Well, you've got a decent-sized community that sells pins to each other. You could have pinsiders document completed transactions something like:
1. I buy a pin from tamoore
2. I enter the pin, sales price, region, condition (something akin to pin rating system) and serial number of the pin.
3. He enters the same details as above..
4. Do some math to normalize our collective assessment of condition.
5. Update moving avg for that title.
It's true that ebay prices and probably CL prices will be higher, but a purchaser can adjust for that if s/he chooses.
By having both parties certify a sale, you have some expectation the data isn't crap. Maybe require that at least one of the pinsider's have some amount of tenure ( 6 mos?) to weed out people trying to scam the system. If two parties try and ping-pong a pin between them to manipulate the price, you ban 'em or exclude them from using the feature.
Something like that might work. Either that or build an extension mechanism into pinside so you can shop out your light work to others. Maybe expose services to you via REST or whatever.