Is this for investment purposes, in which case, how would they sell this in the future? Surely the legality issues would reduce the pool of potential customers, and the overall resale value.
Or are there people who will buy such a painting for their own personal enjoyment? (I guess just because I don't see the value, doesn't mean there aren't people with vast amounts of disposable income who will buy things like this on a whim.)
If that hypothetical person happens to live in a country where extradition to New Zealand would be impossible, or they have appropriate connections, it might be seen as an acceptable risk.
And perhaps in a few generations, the heirs of that wealthy person could sell it on. I don't really know how it works if you discover you've inherited property that was stolen a hundred years ago for example. At what point does it become yours?
At no point does it become yours. I'm pretty sure that, while you can't necessarily prosecute the person currently in possession, you could almost certainly recover the property if you could prove it was stolen from your estate.
Added: In most English speaking countries it becomes a crime to willfully possess stolen property as soon as you know that it is stolen. In general, the lack of mens rea in merely possessing stolen goods would make it enough to repatriate the goods, but if that's not enough, most statutes on the matter explicitly state that you must know and wish to unlawfully handle the stolen property.
Edit: or not in this case, my bad.
In Poland the Art. 292 of Criminal Code and Art. 122 of Misdemeanor Code says something like "an item, about which on the basis of accompanying circumstances should and may be suspected that it has been procured by an illegal act" and directly addressed accidental fencing of an item (Art. 291 and 121 respectively address intentional/knowingly fencing an item).
I have no idea how that works in practice but googling "accidental fencing" in Polish ("nieświadome paserstwo") brings up a few results about people who bought cheap stuff online and it turned out to be stolen.
Here's one case like that with a lawyer's answer/opinion if you want to try Google page translate or can read Polish: https://www.eporady24.pl/nieswiadome_paserstwo,pytania,6,64,...
If it's stolen and the government is after you (in this they will), you're toast.
It's not like buying something from e-bay, and it turns out the seller boosted it from a van; that is not your fault.
I think you may have lost track of the thread here. I was addressing this question posed by gtsteve.
> And perhaps in a few generations, the heirs of that wealthy person could sell it on. I don't really know how it works if you discover you've inherited property that was stolen a hundred years ago for example. At what point does it become yours?
It is obviously unlawful to traffick knowingly in stolen goods. It is as serious a crime as the theft itself (sans aggravating factors like use of force or intimidation, especially with a deadly weapon).
Through my travels, I've met some insanely wealthy people (most were borderline billionaires, some were actual billionaires) and I've been told once you reach a certain income level, having something that is rare or something nobody else has is their way of standing out among their friends. If all your friends have more money than god, you need something especially expensive or rare that isn't acquirable from simply being able to buy it.
Secondly, wealthy art collectors are notorious for buying rare, stolen art. There's an entire underground for it. I've read several books on it and you'd be surprised at how many pieces float around in the underground market without anybody ever knowing about it. I've had friends who run in the art circles comment on a famous piece that was being displayed in a person's home and they ask about it and they brush it off and say, "Meh, it's a replication, I mean, it would be illegal to have the real thing, right?" with a wink and a nod to the person. Just the idea they have something no one else can have gives them plenty of enjoyment. Passing it off as a fake and the thought they're getting away with something also has something to do with it.