It hasn't happened with anything I currently own, but in the past a simple it's not for sale usually sufficed.
There are a lot of times I can't get out of the gas station or car parts parking lot. I'm sure it happens to a lot of folks with vintage/classic cars and motorcycles. Unless I'm in a huge hurry I'll BS with the folks. Most of them are truly interested in the car and are smiling the whole time.
I truly have never run into anyone I thought was rude.
Since my wife is in lust for a '34 coupe, I would sell either of my "babies" if the right price came up.
If I was not interested in selling I would quote a higher-than-market price. Of course, if they wanted to pay significantly more than it's worth I would have to be ready to sell.
PS - It's really fun to take your car to a new car sales lot
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