By default, items are viewable and available to your Friends, and Friends of Friends. This makes it easier to find people that may be interested in items you have, and for you to find items you're interested in.
Remember, you get to decide who or if you give an item away. So if you don’t want to give an item to a Friend-of-Friend, no sweat.
Posts to Facebook
Items that you post via the mobile app are viewable on Facebook just like any status updates on your Timeline or Newsfeed. Just like in the app, unless the person viewing the item in the browser is a Friend or Friend-of-Friend then they cannot Call Dibs.