How large does a market have to be to "legitimately" place a value on something? If there exists one person who will pay you $10 for your used tissues, are they

How large does a market have to be to "legitimately" place a value on something? If there exists one person who will pay you $10 for your used tissues, are they worth $10? What if there are 10 such people, or 100? I'd say the market has to be 1 person, but I suppose the IRS has a different definition.