Depends if you are the buyer or the seller.
The seller wants as much money as he can get(screw the buyer);
The buyer wants to pay as little as possible (screw what is fair for the seller)
Otherwise check the local or regional papers for like vehicle. And take an average price. Everybody thinks they own a one of a kind above average vehicle and ask more than they will get. If they start high, they can go down. If they start low, they can't go up when the haggling starts.