Is it ever moral to take advantage of another person's ignorance?

Suppose that I drop by a yard sale to see whatever is up for grabs. While rummaging through the junk for which the owners no longer see a reason to keep, I catch sight of an item which I know to be extremely rare and valuable. Would it be moral for me to pay the measly asking price of the seller so as to resell it at auction for a much higher price, knowing that the owners have no clue, and having no intention of informing them, about the true value of the item they are trying to sell?