Sorry, not to be a dick but owning one two years doesnt qaulify you to know two cents worth about Miata sale values.
Although you are correct that mods only bring a percentage of their value, you again show your ignorance regarding Miatas and their mods and pricing when you say the Volks are the only item on this car that has "resale value".
I can tell you from first hand experience that I could sell a couple grand worth of parts off that car easily.
As someone who has engaged in buying and selling dozens of Miatas I can tell you that year model car as clean as that one, with that mileage, in stock trim will fetch $5500-6000 easily if properly marketed...and thats with no mods. Add several thousand dollars worth of mods for an additional grand or so and his asking price is right in the ballpark. If someone gets the car for $6500-6900 they will be getting their moneys worth. To be honest with you if it was my car I would strip all or most of the aftermarket parts off it, sell them individually and then sell the car. I would net more then the $7500 he is asking too
Citing some riced out Miata you saw bring a certain price is really proof of nothing. I have seen a 2 year old Mustang GT 5.0, with 40k in perfect shape sell for $1500 before, but thats certainly not the norm.