If there are ten users with the exact same sales (0) - how do you define "bottom 5" out of these ten? And how would you sort the worst customer and the 5th worst customer? This seems to be more of a logical question than a Qlik question, and FirstSortedValue() doesn't break ties in itself.
You could try breaking ties manually and quasi-randomly or arbitrarily, e.g. by adding Rand()/10000 to each aggregated sales total, which should break ties (but still be extremely unlikely to change the actual order of the customers). This will make the five objects populate randomly and re-populate randomly each time a selection is made, which could be kind of confusing to users.
You could also add some sort of arbitrary small value, e.g. assign an incremental number to each user and then add UserNumber/1000000 to your formula. This has the advantage of returning the same result each time but the order of the users will be completely arbitrary based on how you assigned these numbers in the first place.
