I think this being a good answer hinges on the resulting user experience. If one person spends several minutes going through the process of putting in their payment and other info only to be told you don't have a ticket, they're going to be pissed. So you need a good answer on how to avoid that experience, however you implement the locking.

Saying this as someone who has been on both sides of that interview question, and also evaluated performance in it as a hiring manager.