I've seen a few pretty nice ones (But not $150,000!) at one or 2 of the "better" gunshops in the area. I would pretty much bet they were being sold on consignment. Few good businessmen would tie up alot of money in guns that have a VERY limited audience. For the 20-30K a Parker Bros. shotgun, they can buy 40-50 "normal" guns, that they will have a much better chance selling. There are some "specialists" that sell on the web, I found some when I was researching an L.C. Smith for a friend. But I still suspect they were mostly consignment guns, as the inventories shown were over a million dollars, easily.
However, I do live not far from Doug Turnbull Restorations. He has many of these guns come through to be worked on (I've had the quick tour, my buddy has had so much work done there he's on a first name basis with everyone!) Lots of rare and unique stuff. He also has a few guns for sale at any given time, although most are in the below $10,000 range.