At this point in time the main benefit of joining Facebook is to enhance your brands image, communicate with your loyal followers, attempt to go viral and add depth to your costumers purchasing experience.
Link building is such a small factor that joining facebook won't have much, if any, impact on your organic rankings. The following link goes into a little more detail.
Why You Need to Make a Facebook Fan Page for Your Website NOW!
Also i havn't had direct experience with the Garage Sale application but if you can angle your products to match the 'garage sale' feel of the app then def. give it a try!