If you only want registered users posting classified ads, do the following:

1. Go to Classified Admin > Settings > General > Registration.
2. Make sure “Require user registration?” is checked.
3. Click Save Settings.

Now in order to post a classified, your users must previously register with your site and be logged in.  When they attempt to post an ad, their user profile details will be used in the ad submission form.

IMPORTANT:  Users that have blank or incomplete profile information (e.g. missing first/last name) may complain that their ads are being placed with incomplete information and that they cannot edit their ad contact info.  They must have a complete profile to display this information properly.