If, for some reason, you can really only make a lo-ball offer on a car, there's no shame in telling the seller that straight out. Tell them you know your offer is low, but give them your phone number and say it's a standing offer. The key is to just not be insulting about it. As long as you act like a stand-up guy, you may get a call in a week or two and the seller will take you up on your offer. There are as many different kinds of people in this world as there are makes and models of cars.
Some people are simply jerks, and they are entitled to live their lives as they see fit.
But there's no reason you need to do business with them, especially when it comes to buying or selling a beloved car.