Well sometimes people will turn in tickets they can't use and then one of two things will happen:
1. The BO will accept them back and then sell them on the cancellation line
2. The BO will not take them back but will tell them to take someone from the cancellation line outside the theater and sell it to them themselves.
Generally speaking though, the BO will sell seats that haven't been paid for yet before they sell someone's ticket which has been turned in.
And hang on, when did you win the discus?