You control the criteria by which listings are displayed within the category browsing results feature. Highlights of this feature:
<tip c n>If display order does not seem to be controlled by these configuration note that the better placement feature trumps display order</tip>
The site wide default display order is set here in the admin tool:
SITE SETUP > BROWSING SETTINGS > Default Order of "Classifieds" while Browsing Listings SITE SETUP > BROWSING SETTINGS > Default Order of "Auctions" while Browsing Listings
They are shown in the screenshot below:
You can set display order by listing type on a category by category basis within the following admin tool:
CATEGORIES > CATEGORIES > [MANAGE] (next to category to affect) > FIELDS TO USE
Like you see in the following screenshots:
Then click:
and finally below after "category specific configuration" is selected: