If there was a business premises and phone, wouldn't they quote that in the ad, rather than the home address? Which doesn't help to explain why someone on a modest bungalow would be selling a Lotus 18!
"Offering for sale" and owning an expensive car are different considerations. Most of the people who advertise in the back of classic car magazines would be bust if they owned much stock.
When there was a wife at home, it would have been common to use the home phone number for some business. Not to sell an Anglia, but for an expensive car it might be nice to have a lady answering the phone. "He is in the workshop at the moment and can be reached on 23456."