Hi Jack,
In both cases, you need to replace the text in the template for each section by its associated Auctiva tag. Since there isn't a [DESCRIPTION] or [DESCRIPTION_AND_IMAGES] tag in the templates, the one-page lister would reject them (won't allow use). That explains that problem.
As example, on that first practicewriter link, the description section text "You are bidding on the 2004 Hallmark Ornament Yertle ... yada yada ... down to THANKS for LOOKING! where the tag for [PAYMENTINFO] starts that section is replaced by the [DESCRIPTION_AND_IMAGES] tag to allow you to use the image selection and the description fill-in box on the one-page lister, i.e. that's where that text belongs. The tags for the seller details would replace each of those text sections, and you create a Seller Details profile where you store those text sections. On the one-page lister, you select the Seller Details at the right of where template is selected. That coincides with your speculation about payment (needs text removed and replaced with [PAYMENT]. One more...The titles at the top are replaced with the [TITLE] tag.
Hope that all makes sense. If it doesn't, post back questions. This can be "rocket science" for many, so it's not wrong to ask. Sounds like you got a bunch to do, so I'll try to get you going down the right path. BTW....looking at one of Auctiva's templates as an example using the Customize an Auctiva Template can really give you a good guideline.