I sell about one book on B&N for seveb on Amazon including the foreign stores.
The same is true of Apple iBookstore, or a little less.
I'm primarily selling short stories, poetry, and some poetry and story collections. My average is a little over one a day. On individual days there can be from zero to 5 sales. That's why the average is a little over one a day as if one can sell a half a book. Well you know how stats are.