classifieds police?

just nosing through the classifieds and noticed alot of adverts with items that have either been sold or been retracted. i realise the thread starter can delete the entire post but can forget once the item has gone but is there anyone allocated to remove such ads or is there a way that they can just time out? i understand that 20 comments down the item may longer be for sale so you at least know, but if the advert was removed it would save having to scan through it all to find out.
is there any reason they are kept up?