as Drew mentioned your best bet would be broaden your campaign to sell the car as much as possible because it is a niche car that appeals to a small cross-section of the car buying public. Even selling a more modest E-34 in nice shape for less than half the price can take six months. You have a lot of competition in that price range for a high performance Sunday driver as the person that will spend that amount will likely be a collector that knows a great deal about that car...so you will have to be very patient with your sell.
As another data point one can find a pristine '91-92 low miles E-34 M5 with 3.8 liter motor for around $20k if not just underneath.
Again, it is a very nice car and probably one of the finest M5 E-28's around.
Good luck with your sale.
George