SellSimple is a new website and iOS app (free, App Store) that allows you to list your items for sale on several popular marketplaces and social networks simultaneously from your iPhone -- in under a minute. After setting up an account, it's just of matter of snapping a few photos of your, ahem, "treasure," entering a description and choosing a shipping price and/or local pickup option.
Your item is immediately listed on the SellSimple.com marketplace and you have the option to also list in on: eBay, Etsy, Craigslist, Facebook, Twitter and to share it via Text Message and/or Email. I received a question about a test item I posted last night via push notification on my iPhone about a minute after posting it. I haven't yet tried cross-posting the same item to multiple services, but the app certainly has a lot of potential for simplifying what could be a dauntiing task.