I'm guessing most of the high end finished trees (10k plus) are sold through networking. At least that's how my teacher does it. He has an established set of clients that he offers trees to. He recently increased the number of trees imported from Japan, stuff that easily belongs at the Kokufu. The trees are only at the garden for a month before they're shipped off to a client. Rarely does he advertise them publicly.
That's mostly how several high end sellers I know of around here sell trees. I know one nursery that only sells high end trees (over $5000), but mostly cares for those high end trees for their owners.
There is a shadow market, of sorts, for really really good trees that isn't seen much at shows or local clubs. Owners of those trees tend not to want to let people know they have what they have. They're not online either (or at least not very intentionally visible), for the most part.
Doug Paul, who owns the Kennett Collection has an astounding collection of extremely high end imported (and domestic) bonsai, is in that crowd, but he's generous with time and visibility of his trees. He's is also one of the first Americans to have a tree exhibited at Kokufu (about 15 years ago now)
The Kennett Collection is a private bonsai garden located in Kennett, PA. Check our photo report and movie in this article.
www.bonsaiempire.com