Hello again volunteer,
Well, that's kinda tricky. You have to take in consideration that, my above-presented solution, is just an "hiding" solution, and one little tweak about the validation. So, if you want to hide the contact button on the ad page, that's easy. But, that solution will apply to all users. What I mean is, if you "hide" the contact button bellow the ad, it's hidden for all kind of users. Registered or not.
So, think about it, and if this is what you want, just go to your theme folder, open item.php file, and alter the following:
Line 110:
<a class="btn primary item-contact-button" data-controls-modal="item-contact" data-backdrop="true" data-keyboard="true" href="javascript://"><?php _e('Contact seller', 'twitter') ; ?></a>
Comment the line like this:
<!--<a class="btn primary item-contact-button" data-controls-modal="item-contact" data-backdrop="true" data-keyboard="true" href="javascript://"><?php _e('Contact seller', 'twitter') ; ?></a>-->
And that's it. Save the file, refresh the browser. There you go.
But! And this is what I have in mind for you. I imagine that the sellers have to be somehow contacted. As an alternative solution, after you hide all above, you can use the "Custom fields" where you can make custom requests from users. As in telephone number, email, etcetera. Make some custom fields, "TEXT" , and make them non-mandatory, or not-required.
With this solution, the seller will be able to leave some contact information, without being restricted by validation or mandatory fields.
The custom fields will appear under the ads text, and will be visible to visitors. Witch is a good thing, if someone wants to contact the seller.
I hope this is what you are searching for. Have a nice day!