run.sh opens two Postgres sessions and runs the SQL in answer.sql in both, slightly staggered, against a single remaining seat. From the root of your copy, run ./reps/08/run.sh <connection> (or sh reps/08/run.sh <connection>; the top of run.sh shows the way for Postgres in a container). Read answer.sql first, then run it and compare with the expected result below.
Expected with the shipped file: remaining = 0 and sales = 2, for a seat that existed once. Change answer.sql so the seat is sold once, whichever session gets there first, and a sale only happens when a seat is left. A fixed run reports remaining = 0 and sales = 1.