I am very new to Auctiva. I had a hard time figuring this out and it is really so simple. Just go to "ITEM details" (item details NOT seller details), either a new one or edit an old one you have already saved. Scroll down to where "Select template" is on the left. If you haven't alreadt selected a templat, do this next. After you have saved your template...look to the right of where the template selection was. You will then see "select seller details"
I hope this helps. Let me know if you have other questions.