We have to do following two items to make the external link to nofollow.
1. To make the Custom URL to NOFollow:
Step 1: Please take a backup of oc-includes\osclass\helpers\hItems.php
Step 2: Replace the attached hItems.php
Step 3: Test it by placing new ad
2. To make the HTMl Description link to NOFollow
Step 1: Please take a backup of your item.php under your theme and replace the attached item.php file.
Step 2: Place the attached external_nofollow.php file under the theme folder.
Step 3: Plase a new ad and test your website for nofollow links
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This solutions works well in my Free Online Classified website
http://adzhome.comAbove solution is consolidation of many other solution in the forum