Sometime you might want to list inventory for a past event in order to keep your reporting accurate.
By default, the purchase order screen uses the current date as the start date, so if you search for a headliner, it won't show past events. Put the event date in as the Event Start Date field, click FILTER and it will show older events, allowing you to add inventory:
In the Pricing/Inventory part of the portal, you can do the same thingto find the inventory if you want to invoice them out for whatever they were sold for.