I agree there is a major problem in determining populations given that PCGS will not attribute PL buffalo nickels. I have never seen a nickel in a PCGS holder close to mine, but with tens of thousands of coins certified, there is no way to say definitively. As for auction records, there are none so the first person to sell one at a fixed price or who is brave enough to face a no reserve auction will ultimately set the market. The price sounds very high especially for the grade to me, and my personal interest level in PL coinage is nowhere near that for any 20th century coin. I think they are trying to extrapolate from other rare PL series (e.g. Peace Dollars), but it is unclear that a PL buffalo would fetch anywhere near what the PL Peace Dollars do even though the nickels are rarer in PL.
I hope they do sell it successfully for $7,500 but that would be a fantasy for me.