Yapta will alert you when a lower room rate becomes available from the hotel(s) that you’re tracking.  It does not recognize whether a room has been purchased or not.  Unlike airlines (which have “guaranteed airfare” policies), most hotels do not have price protection policies.  Instead, most will allow you to cancel a reservation up to 24-hours in advance of your stay.  So, if Yapta alerts you that a lower rate is available, you may want to take advantage of your hotel’s cancellation policy and re-book at the lower available rate.  In most cases, you will have to book directly with a hotelier in order to take advantage of such cancellation polices.