I don't understand why anyone would want to buy something outside of its legal socio-economic context, when that thing's entire value is derived from that context and is otherwise worth very little intrinsically based purely on aesthetic value.
Any market value it sells for is then based on some shared mental deficiency which prevents people from understanding that.
And, a thing's economic value is derived from what someone else will pay for it, and only that. If someone else (or several someones) will buy it, whether or not the law agrees that it's legal, then the thing is valuable. (See drugs for an example.)
I wouldn't say that such people have a mental deficiency. A moral deficiency, yes, absolutely, but not a mental one.
One owner who knows the value but won't sell at that or any value, one willing to pay a thief less than that value, the thief, and a buyer willing to pay more than the thief was payed.
"Wow, the museum payed a million dollars for this? I'd be willing to pay 5."
"Oh? Suppose I know a guy who'll nick it? Now, it'd be stolen so I couldn't charge quite as much, but the guy takes his cut up front, so let's call it 2?"
"Really? Gee, that's a steal"
cue: looney tunes