The market on Z31s have slowly been increasing...clean 87+ turbo cars are now regularly going for $6-8k...they only made about 8,000 of the restyled turbo cars and 1000 of those were Shiros.
I have also seen super clean heavily modified 400 hp cars (properly built) go for over $12k...it is not at all odd to see these cars once properly modified to appraise for $12k+. I'm not sure how many people are going to pay that however, the market remains small...it seems most people would rather by a Z32 than a Z31.
All that aside I'd say that this seller would be lucky to see $10k for that car...in factory trim these cars really are not all that special by today's standards...it's the high potential that holds my interest.