I put this feature into Auctiva Poster when I programmed it in 1998 and almost nobody used it. I called it custom items I think. Hmm, it's still there in Auctiva Poster. If you go create a new template you can add fields.
Then when you are creating listings using that template there are boxes for you to fill in for each of the fields you created. They are saved per item. So if you had a clothing template you could have those fields and enter them per item, and they would be placed in your template where you wanted.
A very cool & useful feature, but too seldom used.