Read this article if you noticed issues with changing the author of the ad in backend.
Recently (after Joomla 3.6.2 update) the issue with changing the ad's author appeared.
If you cannot change the author of the ad in backend and this does not work for you:
you can fix it by replacing the file located in:
with this file: DJ-Classifieds - changing author of the advert Patch (11.19 kB 2016-08-16 14:42:23) (download available for all active subscribers)
If you have any questions, let us know in comments!