I don't know your model, but if you go a such granular level, you won't be able to show overall sales. You can create a variable in your script that holds the sum of all sales and then use that, but this might not be useful if you are working with different markets.
I have not tried this, but you could create a aggregated 'total sales' value in the data model that all sales people have access to in addition to their own values. That way you have both their detail and a grand-total value available without the detail of other sales person activity.
I had the same needs on a couple of documents. I (kind of) solved it by creating 'hide prefix' fields that I set on open by using the OSUser() function and then locking them. It's not foolproof and definitely a work-around, but it's seemed to work so far. I wrote up a summary at Fortune Cookie BI: The Magic of the Princess Bride . (Pardon the random Princess Bride quotes; I do that so my designers will at least look at the posts).
If anyone has any thoughts/comments/criticisms I would love to hear them!