If you need to clear out the inventory of a particular custodian, or for any other reason select all inventory in a custody in an Event, the 'select all' button is the way to do it.
1. Open up your Event and navigate to the Mark Used Inventory window.
2. Apply any filters you would like in order to pull the correct inventory into your Search Display
3. Next, click "select all visible" on the right above the results window. 
4. Then determine if you'd like to select just 1 of each device or the entire quantity of each device. 
5. Depending on your selections, this will move all of the selected inventory to the "Selected" tab. 
6. If you opted to select the entire quantity of each device, then all of the inventory visible in your 'search tab' will be moved to the 'selected tab'. This will also cause the search tab to refresh with another list of available inventory.
7. If your intent is to select everything in the current custody/filters, then repeat the process of clicking "select all visible", until nothing is returned in the Search tab.