Just my 2p worth, I think the current state of the market is also a big factor, with petrol prices shooting up (supposedly to ?1.50 a litre by the end of the year) and all the extra taxes the government keep dreaming up, it is becoming an expensive car to run (on top of the usual maintenance of a 10 year old turbo'd motor) so far fewer people are looking at buying any kind of sports car at the moment.
I got myself the cash to buy an S15 / EVO / Skyline around 6 months ago then spent some time trying to come to a decision as to what I was going to buy, but in that 6 months things have changed so much that I'm now looking at diesel cars just to ensure I can afford to run them and that they're actually going to be worth something in a few years.
As said above 10-12k for a 9-10 year old car with 70k on the clock seems quite a bit but if you have the luxury of waiting I'm sure you'd get the right price for it (providing the 10 year rule doesn't mean an influx of cheaper models) - the rarity of the car means you can demand a higher price, it's just that your markey is unfortunately very limited.
& like you say if you're putting 1500 miles a month on it then even less people are going to be interested as time goes on.
Good luck with the sale! :thumbs: