An answer from e-mail support.
How do I remove the BUY NOW! shopping cart button and replace it with a CONTACT SELLER button that when clicked goes to the internal message system?
To do this, you would open the file /templates/pro/modules/skin_classified_ow.php. Right at the beginning of the file, you see three entries like $skin['price_holder_full'], which correspond to the price tag and buy now button areas. Remove the line (#34) with "<img ... button_buy_now.png ... />", and replace it with the following:
<a href="javascript:null();" onclick="putToCenter(500, 500, 'general_ajax'); showLoader('general_ajax', 'ajax_500_loader'); submitAjaxGet('general_ajax', 'message&action=load_form&target=member_[[POST_MEMBER_ID]]&member=[[POST_MEMBER_IDENT]]'); return false;">Contact Seller</a>
You can also find the above snippet in the skin_members.php file in the same folder (with the POST_ prefix in the data tags being the only difference). The above would display a text link. If you wanted to use an image, you would simply add the onclick="..." action above to an image of your choice to open the internal message window for the seller.
