Hi Stewart,
When you create your Auctiva custom template, you implant the seller details using the Custom tags for same, e.g. the payment section has a corresponding payment tag [PAYMENT]. The tags are then used by the one-page lister in building the listing with the selected template. You can also create a Profile for Seller Details and apply the profile on the lister to quickly integrate that info into a listing. Note, use of that Profile on a listing requires selection of a template (and the template must have any/all of those seller details tags to work). When you first select a template, it gives you a snapshot of the layout including the template's placement of each of those sections for use. Auctiva's ready-made templates already have the tags.
Unless there has been a recent change, that's how I've used them.
Hope that helps.
Danno