If you have a work which says you win ties for a type of sales, and everyone has zero of that type, do you win that tie of zero and get points for the cards in your hand of that type you had zero in?
Just going off how this kind of thing works in most games. I would think you actually NEED at least 1 sale to qualify for interest on sales.
I know it is not stated as such in the rules, but that is how it works in almost every game that does something similar.
Plus it makes sense. How could you earn interest on sales of a product if you didn't even make one?