A price that is unknown is simply "unpriced". Every sale is between two parties, the price is simply any agreed value that would result in a sale. So it's not

A price that is unknown is simply "unpriced". Every sale is between two parties, the price is simply any agreed value that would result in a sale. So it's not just what someone is willing to pay, but also what someone is willing to sell for. If both those ranges don't overlap, no sale. I would argue a better definition of priceless is one in which no value could ever be discovered for which a sale could exist. Not because it is too high, but because there are constraints to that price equation (as governed by both parties) that overlap in such a way as there is simply no solution. Examples are obvious, but in the case of the the Crown Jewels, they are priceless because of their identity and their inseparable significance to the United Kingdom. There is no price for which decision makers could agree on and keep their jobs (or their head), to the extent that a sale could ever exist. In the eyes of the public, the Monarchy itself would be at stake. In that sense the Crown Jewels are priceless, literally without price. If this painting were genuine i don't think there's anything that would stop the seller from being able to agree on a price that would enable them to sell it.