well that all depends on your invisionboard sorry can not be of much help I do not own or use that forum
Basically you need to edit whatever template is that postbit and place some html link there and find out what the invision userid is there and use that
the link is something like this
<a href="http://www.domain.com/classifieds/showcat.php?cat=500&ppuser=SOMEUSERID">User's Ads</a>