I would have absolutely forced the issue w/ them. I would have taken a picture of the sign (even if w/ a CP camera). Your position should have been that the computer system obviously hadn't been updated to reflect the latest pricing, and they were obviously the ones making the mistake. Since they're out of a job anyway, I'm surprised that they even cared. I would have then called the police and informed them of the situation.
Also, if the liquidator has already purchased the inventory, it's up to them to set the pricing. How is it a 'mistake'? I really would have pushed it further. I would have wanted to speak w/ the top-level manager. If he would continue to stone-wall you, at that point in time I would have informed them that I've already contacted the police, and that they should be arriving shortly.
I've worked in retail before, and what usually happens in cases like this, even if it's a case of mis-pricing an item, is that you sell them one item at the stated price, and promptly remove the signage. I believe there's a similar law here in NJ regarding B&S pricing. Car dealers were notorious for it. They got around it by listing one specific car (listed the VIN for the car) at the low-ball price, and of course, that one would have been sold by the time you get there. However, they have several others at only 'slightly higher' prices.
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