Jim goes to B&Q to get some insulation wrap for extra protection on No. 1 Son's water pipes. Finds last remaining roll of said wrap. Takes it to checkout ans asks 'This hasn't got a price on it, how much is it please?'. Checkout boy scans it. 'Nineteen ninety nine'. Hmm, thinks Jim, that's a lot; I'd better check with Sarah. Sarah says 'Best get it now regardless; much coldness on the way and don't want the poor little chap's pipes freezing like they did last year'.
So Jim goes back to B&Q, collects roll of wrap and takes it to the checkout.
'Can't sell that' says the checkout boy.
'Why not? You've just told me it's £19.99; look, here is a crisp new £20 note that I wish to exchange for it'
'Nah, look, the packaging's damaged. Can't sell that'
'I'm not worried about that - of course, it would be great if you reduced it, but I'm happy to pay full price.'
'I can't sell it cos it's not on the system. It's been taken off the system to reduce the price. It'll go back on the system when the sale starts. I can sell it to you then'
And thus it was that Jim took up the twenty pound note that he had been so willing to add to the coffers of Messrs Block and Quayle (for such were the names of the founders of the business), and took it to Homebase instead, where they sold him a nice roll of insulation that didn't even have any packaging to be damaged.