Is a fair exchange always not a robbery?

There is that saying, “fair exchange is no robbery”, but is it? If a person possessed something that had a high value in a general sense and they knew it but did not care, and sold it cheap to dirt cheap to someone who know the value of the item, and was aware the owner didn’t care, is it still a fair exchange? What if the person who possessed the item was unaware of the value when approached by one who did, but who was also aware the owner did not know, thus getting the high value item cheaper, that makes it less fair and more fraud?