Hey Arthur,
I did test it on http://www.iautosoftware.com/demo/ (v5.0) for user "DEMO".
Same result; Listings NOT sortable for the MakeModel field in "My Listings" ...


As you can see; Photos, Year, Mileage, Price & Posted is sortable. Only the Make/Model field is not working.
In mylistings.tpl (v4.7.1)
{if $property.MakeModel.is_sortable}
<a href="?restore=1&sorting_field=MakeModel&sorting_order={if $sorting_order == 'ASC' && $sorting_field == 'MakeModel'}DESC{else}ASC{/if}">[[FormFieldCaptions!Make/Model]]</a>
{if $sorting_field == 'MakeModel'}{if $sorting_order == 'ASC'}<img src="{image}b_up_arrow.gif" alt="Up" />{else}<img src="{image}b_down_arrow.gif" alt="Down" />{/if}{/if}
{else}
[[FormFieldCaptions!Make/Model]]
{/if}
So why does
{if $property.MakeModel.is_sortable} return
FALSE ?
Even when removing the
{if} statements the fields still wont sort as expected.
The function works perfectly in other search results:
In search_results.tpl (v4.7.1)
<a href="?restore=1&sorting_field=MakeModel&sorting_order={if $listing_search.sorting_order == 'ASC' && $listing_search.sorting_field == 'MakeModel'}DESC{else}ASC{/if}">[[FormFieldCaptions!Make/Model]]</a>
{if $listing_search.sorting_field == 'MakeModel'}{if $listing_search.sorting_order == 'ASC'}<img src="{image}b_up_arrow.gif" alt="Up" />{else}<img src="{image}b_down_arrow.gif" alt="Down" />{/if}{/if}
I believe that the Make/Model field is the most important sorting field for any user that lists multiple cars.
If a user wants edit his/her listings , they wont search for the listing by the number of photos or Mileage.