From our own expeciance we know that many many crooks have their focus on the high priced watch auctions. For example at one recent bid on a vintage Zenith Auction I did not win (was second high bidder) I received 3!!!!!!!! emails from 3 different parties trying to offer the watch to me as the high bidder backed out. I can easy imagine that a seller of high priced watches does private auctions to avoid this.
And I somehow also guess that the winner of the 9,1k Monaco prefers to keep his identity secret.
In the Premium segment private auctions have their eligibility I think.
On some other sellers who do private auctions for watches in the +-1k range I agree, their private aucctions are suspicious to me as well pretty often.