I would say there are two things to address here.
First is the motivation for holding back those tickets. There are many legitimate reasons why you might not put a ticket up for sale. It might have to do with production requirements of the event. You're not sure if the patron would be able to have a clear view of the stage for example. There are a variety of practical considerations.
There are practices in the industry of holding back tickets to manage the pricing. Fundamentally—