The way it works is this. Lets say I have a reservation for Be Our Guest and I am going to cancel it, I will post it on here. You would then send me a PM (Private Message) so we can coordinate a time for me to cancel, and for you to hopefully pick it up before someone else does. Usually people will give you a heads up for the more difficult ones to get. Others posters may simply cancel the reservation and just post they cancelled it. Like I just did. I saw times available still so I simply cancelled mine and posted that I cancelled.
The very first post to this thread will give you more guide lines as trading reservations is no longer allowed by WDW. Also, all cancellations in this thread are for December 2012 only. There are other threads for the other months.