Apparently M&S was selling 20 different product lines for a penny each yesterday to mark the company's 125th anniversary. Queues were out the door and down the street.
Well of course they were.
If you sell something that's worth a fiver for a penny, the back of the queue for it will be populated by people who value the time they expect to spend queuing at £4.98, gaining almost nothing. (Only the imperfect dissemination of information and travel costs prevent this from happening instantly.) It is of merely academic curiosity whether you'll run out of cut-price items before society runs out of people who value their time at such a low rate as to make it sensible for them to join the back of the queue - yesterday, M&S happened to run out first.
There is though, one clear PR benefit to the exercise. When Ikea tries the same promotional tactic, a violent mob normally form at the front doors. With its customers forming orderly queues even when offered goods for a penny M&S has at least confirmed its reputation as the most genteel of British retailers.