Just a though from someone who has a decent amount of random parts and always seems to be looking for parts too.
A "wants" list for parts desired and a "gots" list for parts a user is willing to part with. If you are looking for a potential example check out any beer on beeradvocate.com as they have a wants/gots system for beer trading.
I suppose the challenge would be in creating a database of parts.
Thoughts,
Ed