I'll give my opinion - if that's what it is. We've talked about this a good number of times over the years.
I leave the posts up until asked by the seller to remove 'em -- even if the item is sold. Yes, it does present an inconvenience now and then (from someone trying to buy a piece that was sold long ago), but we've chosen to do this to allow those ads to be an archive (fancy word, hey?) that serves as a price guide, if you will.
And that's it. I'm open to changing the way we do it. But -- there's always this just waiting to happen: