In Dallas GS400 are very hard to find especially in black. I just bought an 84,000 mile one in immaculate condition for $16,900, it already had the 90,000 mile service done, brand new tires, new ball joints, redone suspension at 60,000 miles, borla exhaust, and aftermarket xenon lights.
It also has some rock chips. I think you should be able to sell it on ebay with a no reserve auction for a few thousand more than 12k, but maybe I overpaid.
Black GS400's last about 2 weeks at most in the Dallas market.