It sounds like you need to research Filtered Portals. One portal can be related to the retailers for that Sale rep. Then filter it to show only a "chosen" Territory (via drop-down, perhaps). It will change the records shown in the portal. This is done with a calculation in the portal dialog (select the Filter button to bring up the calc dialog). You can set the filter to show all in the relationship if the Territory is not selected, or just those in the single territory.
Thank you Beverly. The issue above turned out to be that I had made a mistake in the key fields, and that's why I wasn't getting the correct linked records.
Concerning filtered portals, I do use them, but with this application, as I have up to 4 different search fields, I thought it would be easier filtering thru a relationship(s) than thru a portal.