The simplest way to do this in FileMaker is with a sub-summary report. Just create a Summary field that's the average of sales, and put it in a layout sub-summary part, sorted by seller, and the same field in a leading or trailing grand summary. Now sort your records by seller and you're done.
A subsummary report is good for printing.
Otherwise I would do two portals side by side with different filtering. There would be two global fields to pick the sellers, and filter the portal.