[[!getResources? &includeContent=`1` &includeTVs=`1` &processTVs=`1` &tpl=`product_navigation_left_main_TPL` &parents=`7` &limit=`0` &sortbyTV=`product_type` &sortdirTV=`ASC` ]]
This question has been answered by sottwell. See the first response.
As far as I know, you can't. You'll need a different getResources snippet call for each section.
Use a snippet that reads the tags, and for each one sets up $modx->runSnippet('getResources'...)
You can have it generate the heading from the tag and set it as a placeholder, and set the output of the getResources run as a placeholder as well. That way you can use a chunk to lay it all out.
There's probably a better way to handle the whole situation, but this is what comes to mind right now.
[[!tagLister? &tv=`product_type` &tpl=`product_type_tpl`]]
<section class="tagpart"><h3>[[+tag]] ([[+count]])</h3> [[!getResources? &debug=`1` &includeContent=`1` &includeTVs=`1` &processTVs=`1` &tpl=`product_navigation_left_main_TPL` &parents=`7` &limit=`0` &tvFilters=`product_type==[[+tag]]` ]] </section>
<a href="[[~[[+id]]]]" title="De Bruyne: info [[+pagetitle]] | product by [[+tv.product_leverancier]] for [[+tv.product_type]]" class="[[+tv.product_leverancier:replace=` ==-`]]" id="[[+tv.product_type:replace=` ==-`]]" >[[+pagetitle]]</a>