I've had a ply around today and I'm not sure exactly what was preventing it displaying a true error message yet, but I've done the following things:
Added a global free postage as well as a personal postage for the user you setup with me.
I created an address for the seller.
I added a currency (vital).
I unlocked on the forms page the Specifications, Postage and Stock sections, though I think it was the Postage and Stock options that were the crucial ones. I also added a default postage option. I think that might have been the major issue.
Added a specification and options to it and then linked them to the categories.
I think we need to create a better way for admins to customize Classifieds so that the setup process works more smoothly. Have a play around with deactivating what you don't need out of those and get back to me if one creates an unanticipated issue.
I know there is at least one error message not displaying, so I'll take a wander through the error messages and see which is missing and try and get that sorted.