One other angle to consider... Maybe they are selling off possessions someone else wants/needs for their collection. The question I have is - do the buyers have something tangible the Rothchilds want or need, or even a "service" they need?

To make this more clear - think of it like the NFL draft. Team A has a pick or a player Team B wants. Team B offers them something and it isn't enough, so Team B sweetens the offer with a few picks for later draft rounds. Maybe the Rothchilds need something someone else has and letting Art go to them is their way of sweetening the deal.