You could create a calculated field - median([Sales]) and the sales peoples names. Read more about calculations here: http://www.clearlyandsimply.com/clearly_and_simply/2010/10/calculated-fields-in-tableau.html
Or you could change the level of aggregation on the chart. Right click the pill on the marks field, and change from sum to median:
Please note that the median aggregation is only available in this case because the data source is a Tableau extract.
Tableau will not offer median as an aggregation if the data source does not support it.