As far as I know the sorting is not done on the archive page template but in the settings for the classifieds or places.
As an example for classifieds:
Go to classifieds > settings > and then selectt sorting (second tab from the left).
Once there on the left it says 'fields' and on the right is a column 'sorting options'.
I only use 'add date'. If 'add date' is added to the right column click on it, and the options are shown. Now select the option 'descending' and it should be working. I also marked it as default sort.
Don't forget to flush your cache after changing settings to make them work immediately.
