Selling Priceless Items

As part of something I'm working on, I've run into the question of "how much can you sell a priceless item for?" If you come across, say, Excalibur or the Ark of the Covenant in the course of an adventure and you want to sell it, the question becomes less how much people are willing to pay for it and more how much people are capable of paying for it. This is different from selling a large hoard