Maybe start a blog? Lots of sites use this to get traction.
I guess to clarify a little we see two sides of the equation: We want to reach people in NYC with a large volume of things to give away (movers might be a perfect example). We need to a) reach these people at a time when they're in the mindset for passing things onto others, and b) equally have enough people who're keen to pick things up so the transaction's as efficient as possible. Classic chicken-egg :)