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Updated the original post with info about mileage. If anyone is interested your can recreate the regression analysis in R using the data and the following code:
library(xlsx)
porsche <- read.xlsx("Porsche 964 Price Analysis.xlsx", 1)
porsche.clean <- porsche[1:425, 1:5]
porsche.regression <- lm(Purchase.Price~Mileage, data = porsche.clean)
summary(porsche.regression)
plot(porsche.clean$Mileage, porsche.clean$Purchase.Price)
abline(lm(Purchase.Price~Mileage, data = porsche.clean), col = "red")
I know you are talking about "average" price. Your averages date back to 1991. I think if you ran the numbers from 2015 to 2018, you would have something completely different. 964's are selling in the 40K-50K range now.
As noted, there are plenty of private party sales that are not publicly advertised. I sold an '88 Targa to a neighbor who had previously asked for dibs. We agreed on a price, shook hands, exchanged keys & title for payment. The car was never officially for sale. I don't even think he told his wife... yet. Unlike real estate there are no public records of the transaction (maybe sales tax/registration but likely not public).