I saw the TV special they did for Ron Pratt's collection. If you listen real closely he gives an indication the collection was sold to fund his passion for vintage planes. I don't think the selling of Ron's collection is necessarily the single indicator the market is down. Yes, BJ's revenue is down, I do not think they will go hungry or stop doing auctions. Overall, I think a lot of buyers finally have what they want. Detroit muscle cars, as cool as they are, they are not rare. Every auction has no shortage of them and to be honest, after awhile, the excitement fades and prices reflect it. Truly rare and unique cars will always hold their value but another '64-68 Mustang, another 1st Gen Camaro, another Chevelle is just that, another car. Cars like vintage Ferrari's and Lamborghini's continue to appreciate. More people are buying up nice examples of models from the 80's and 90's now, just look at Countach and Ferrari 308/328 prices. Air cooled Porches, same thing, just going up. Will these cars ever hit a cap, probably but no one knows when. All the big pre-war stuff, I think it is beautiful but I am no expert so I cannot comment.
Since this thread is about classic Z prices, I will give me .02. Thanks to all the media outlets, car websites, magazines, and the release of new cars that remind people of the classic Z's (FR-S/BRZ) there has been a constant spot light on the Z cars for the past several years. Just looking at 240z's, from a collector stand point, there are a lot of them out there. Which is good because at any given time there is something out there for every level of Z buyer, from pristine restored cars, to low mileage originals, to very nice drivers, to projects, and everything in between. Stock/Stock-ish cars and tastefully modified cars will always do well in the market. For a clean 240z that is stock and well maintained but not necessarily restored is a $10K - $15K car all day long depending what part of the country you are in. Truly restored cars start at $22K and seem to cap around $30K with a few outliers that have been bought at auction. Just average cars needing some work but could at least be driven home within reason, $4K - $8K. Of course every car needs to looked at up close, consider rust and other issues, changes that are bolt on vs. permanent, everything else that goes with buying a 40 year old Japanese sports car. Things like one owner, low VINs, and popular colors can add a premium to prices across all conditions.
The great thing about classic Z's is that unlike so many other vintage cars, they can be driven and regularly. They can keep up with modern traffic and they don't shout "look at me" as a lot of other vintage cars do on the road.