I've never heard GPs aphorism before, but I "never buy a car from a mechanic" quite often. In my experience, mechanics will either have a really nice car, or a real beater (or one of each, since the nice car is a project car).

It makes sense really, if you spend all day fixing shitty cars, you're not going to want to go home and fix your car, and you're not going to want to pay someone over double your hourly pay rate to do it for you.