If you have $400 million in funds whose origin you can't account for, you move it into some opaque account to buy the painting for that $400 million. The buyer (you) remains "unknown" but now you have $400 million that you can account for, since it came from selling a painting.
In the art auction world, either or both sides can remain as anonymous as they wish, which makes this whole scheme possible.
In the art auction world, either or both sides can remain as anonymous as they wish, which makes this whole scheme possible.