Here is a website that keeps track of Boxster price trends.
Porsche Boxster Price Trends - CarGurus
Of course it doesn't break down the cars by mileage and features just by year of manufacturing. The listed prices are closer to 10k for early model 986s. Personally I wouldn't sell for under 10k.
As a note, year over year listing of number of 986s for sale are down 20%. If that trend continues for another 5-10 years, there won't be many left for resale.