This has come up several times before.
The site admins have said before they like the effectiveness to be shown by leaving the successful sales up.
But the original POSTER can edit or change the title to SOLD as part of the post title. That would save everyone a lot of wasted time looking at items that are no longer available.
If the item takes a while to sell, the original poster can not go back into the original post and modify it. You are essensitially locked out of your own post after so many days. You can give it bump and write sold but some people are dense and just don't get it.