I want to list_authors with a post count but excluding a category

I have a resource exchange blog that a community of us uses to pass on used resources (mainly books). When a book is sold or given away, we attach the ‘sold’ category to it. Posts in the ‘sold’ category do not appear on any view except when looking at the sold category page.

I have a list of authors (sellers) in the side bar with their post count. It counts *all* their posts, but I don’t want it to count the posts in the ‘sold’ category.

I have the same problem with the category list. At the moment this is overcome by asking authors to also remove all other categories except the ‘sold’ one, but is there a better way?

Can anyone help?