The Inspection Support Network has a great report that will provide assistance in marketing to inactive buyer's agents!
This report selects agents who did not have inspections in the first date range, but did have inspections in the second date range. Inspections must not be deleted, canceled, nor incomplete to qualify.
Navigate to Reports -- My Reports -- Agents & Agencies -- Inactive Buyer's Agents
The following page will then allow you to enter your report criteria and build your report.