Gamification, badges, recognition, social profiles. Offer some sense of gratification when they complete a transaction and give something away. Integrate with Facebook, both for inviting more users and for posting on their timeline when they have given something. It highlights their generosity. Focus on having the recipient give feedback to the giver - recognition, express gratitude, how much they enjoyed what they got.

Craigslist has the audience but no sense of identity, community. eBay has the feedback loop and score but no social network.