If you're serious about the cars, I would politely explain yourself to the seller, make your offer in terms of " this is the best that I can do " and leave your contact info.
Probably not worth the breath expended trying to explain to them the realistic value of the vehicle, unless they ask... and maybe not even then.
About 10 years ago, I almost bought a '24 Model T coupe... I 'd had my eye on it for years, and happened to pass by the day the For Sale sign went on it. It was an older amateur resto, but a good candidate for a driver.
The owner started it up, let me take it around the block, and so-on. We finally started talking price. He said "We were asking Twenty-four, but we'll take twenty-two.." Well, little naive me, he was talking Twenty-Two THOUSAND and I was hearing "Twenty-Two HUNDRED"... I didn't realize this miscommunication until I was looking at the receipt he had given me after a $200 deposit... my balance was $21,800.00.
I had given the deposit on a Saturday, intending to come back with the balance ( that I thought was going to be $2,000 ) the following Monday when banks were open.
I agonized over how I was going to get out of this... and finally wound-up telling the owner that "I couldn't make it work, my wife went through the roof, etc".
He was understanding and gave my deposit back with no further discussion. ( I did not bother trying to explain that their price was out of this world ! )
About two years later, I got a call from the owner, letting me know that car was "Still available and they had come down to $18,000." I politely declined.
Two more years later, I got a call again, informing me that "I could have the coupe for $5,000"... I thought this was still a bit high, but getting into the realm of reason.
By this time, I was really not in a position to buy another car, otherwise I would have probably made a counter-offer and wound-up buying it.
I heard that it finally sold for around $4,000 and went to a good home.
Long story, point being if you make an honest offer and sit back, you might be surprised.
It would take a very talented diplomat with a heavy dose of psychology to successfully explain to someone why their $25,000 "Barret-Jackson" car is really only worth half that , or less...
Herman Munster makes a good point too, to have cash ready and also a trailer to bring it home the same day.