When an advert 'For Sale' (or Swap, Free etc.) is complete it's a good idea to mark it 'Sold' to prevent other people contacting you about it. Find the 'Sold' button and click it (shown below). It will change the prefix to 'Sold'. If the buyer pulls out you simply click the 'Unsold' button...
www.thestalkingdirectory.co.uk