I'm not even pretending to know or care about used skyline values in japan, but 3k seems pretty reasonable for any car with reasonable mileage made in the last 15 years. HOWEVER, if you have "heavily modified" and likely heavily "customized" this car, then you shouldn't be at all surprised that you are going to have to wait for someone to come along with the exact same tastes as you. If you can't sell to japanese due to language barriers or whatever, that would make it even worse.