I agree that it is nothing more than common courtesy to reply saying that the item has been sold. Unfortunately, this, to my surprise, does not seem to apply in other situations, eg my daughter on applying for various jobs has not even had the courtesy of a reply saying that she has failed.
To my mind, this does not show these organisations in a good light. Maybe this is symptomatic of the increasing rudeness one finds.
Regards,
Peter G. Shaw