Best way to buy a used iPhone 3Gs?

I lost my iPhone on a New York City bus today and I doubt there's any chance I'll be getting it back 😟 I belong to AT&T and unfortunately my contract isn't eligible for renewal and won't be for another year. I don't want to pay full price for a new 3Gs or a 4G, so I am thinking about buying a used phone to last me for another year, until I can renew my contract and get a new phone. Does anyone have any tips or advice about where to buy a used iPhone and what to lookout for? I'm a little bit nervous about buying from Craigslist or Ebay, but it seems like these sites may be my best option.