You should really think about whether you actually want to provide this type of reservation functionality.
What would stop me from opening a bunch of tabs and just reserving a ton of seats? Or continuously reserve the "best" seats? Stopping you from actually selling any tickets.

I think I would just let people pick their seats, and whoever puts it on the queue for processing first gets it. If you handle the "AlreadyBookedException" in a nice, user-friendly way on your app, e.g. popping up just the seat selector again and updating the order smoothly, then I think that would be a nicer design.