Hi Jerome,
Thanks for that, I decided to do a template override for the toolbar (I didn't see there was one before) as it is the way I am most comfortable with.
Here is my solution in case someone else need.
<? if(JRequest::getCmd('layout')== 'listing' && JRequest::getCmd('view') == 'productmarket'){ ?>
<div class="btn"><a href="custom-ulr-/for-custom-button"><span class="btnName">Custom Button Name</span></a></div>
<?php } ?>
I also changed line 34 to echo '<div class="btn '.strtolower(str_replace(' ', '-', $tool)).'">';
this adds the tool name as a class to the btn which helps to style them differently.
Thanks