> the number of people willing to purchase a black market painting worth ~$millions is probably like a dozen

You only need one.

With something like this I imagine you could hold on to it for years or even a lifetime waiting for the right connection to come along. Maybe some of them are even commissioned or pre-sold.

I can’t say I know anyone with stolen art, but over my lifetime I’ve known people across a range of wealth classes who got a thrill out of acquiring and owning something that was risky contraband.