Seats that cannot be oversold
If your webinar has fifty seats, fifty is the most that can ever be sold, however many people click at once. The limit is enforced in the database, and it is tested with simultaneous purchases rather than assumed. When a seat is held for a buyer who is still paying, it counts against the limit until the hold expires, so the last seat is never sold twice.
If a buyer pays and the seat turns out to be gone, they are not left with a charge and no ticket: the system reconciles that case and tells them plainly what happened.