Hi there!
I'll take a stab at this...
To select a template, whether one of Auctiva's or a custom template:
Open:
Listings (tab at top of Auctiva page)
Open a listing, saved or whatever.
Scroll down to where you see TEMPLATE
Click on Choose Different Template
new window opens
In the upper left corner select Custom under Select Templates
Choose the template you created
Another window opens, previewing the template you chose
Save template to listing
close that window
The new template will now be applied to the listing you are working on.
I do not know if you can change the template on an active listing, but it will be applied to any future listings, provided you have selected that template in the listing editor.