'Let the buyer beware' cannot and is not reduced down to the mere visual appearance of an item. It is not a legal standard at all as your added caveat and qualifications further admit.
also it has to be what the "average" car owner can be reasonably expected to know...so that's open to interpretation too...
it is a legal standard and a valid legal defence.
just that there are also other legal standards that have to be met too
eg:
scenario 1)
my car ad states it has a whine.
buyer comes and checks it over...and buys the car.
a week later the timing chain snaps and ruins the engine.
caveat emptor. not being a mechanic, i couldn't have reasonably known, or diagnosed what the fault causing the whine was.
now on the other hand...if i stated in the ad "drives like a dream" (always avoid ads that state this...lol)
then buyer can say that the product was misrepresented...as i had clear information advantage (an abnormal whine) that was hidden from them
misrepresentation of goods trumps caveat emptor
scenario 2)
i brought the car to the dealer and the dealer told me it would cost £x to fix the timing chain. i sold the car instead saying that there was an impending timing chain failure.
buyer has no recourse legally.
caveat emptor applies. my ad wasn't misrepresentation as i did list the exact fault.
but if i listed the ad with "has whine" and the buyer buys the car and brings it to the dealer...dealer says: omg you bought this car? i serviced it last week and noted the impending timing chain failure.
buyer can now say that the car was misrepresented on the ad as i knew exactly what was wrong with the car. and if the said buyer had this information, he wouldn't have bought the car.
misrepresentation of goods trumps caveat emptor