...a few minutes after the event goes on sale?
When customers find tickets either online or through any of the other distribution channels, those tickets are actually reserved while the customer considers the purchase. During that period, which lasts for only a few minutes, no one else can view or purchase those tickets. If the customer decides not to buy the tickets, they are "released" and become available to other purchasers. So if a customer is looking at 10th-row-center seats and decides not to buy, those seats become available for another customer.
...days after the event goes on sale?