"..In the meantime she noticed that the sign that was displayed was expired"

Was the date on the sign? If so, had you seen it? It does seem to me that the date would be the deciding factor, no the fact that the signs were still up, if it was indeed there.

Since the sale had expired, I'm not sure why the price check policy should apply. I agre with other posters, them offering you the sale price seems like a good deal.