When buying a used car, one of the best pieces of advice I can think of is to take a look at the owner’s manual.
If the manual looks like it has never been opened or is no where to be found, pass on that car. It is not likely it has had proper care.
Joseph, I Agree To The Extent That Finding The Owner’s Manual In The Car Is A Good Sign.
However, a greasy, well worn manual could be a sign that a former owner was having car problems.
You’d never tell the maintenance history of my cars by looking at the pristine Manuals in the glove compartments. I order factory service manuals and second copies of the owner’s manuals for every car added to our “fleet” and they are kept in our home library. Those are the documents with signs of use. That’s where I study and research repair / maintenance issues, not on the driveway.
Ask to see the file I keep on each of our cars and you’ll likely get a better picture.
The future is here now. Some of the cars (Hondas ? others ?) are being sold now without maintenance schedules it seems (according to complaints seen on this site). It’s likely that the manuals in these cars won’t have signs of wear indicative of maintenance.
CSA
Generally speaking, I agree with you, Joe.
However, there is one small flaw in this line of reasoning. A car could have been well-maintained, but the owner might have decided to strip the car of the Owner’s Manual (and possibly other items) in order to sell it on e-bay.
I would suggest that those buying used cars look for maintenance records, as that is the only way to see if the car has actually been maintained in accordance with the mfr’s maintenance schedule. And, of course, comparing those records to the maintenance schedule in the manual would be necessary in order to verify matters. But, as you pointed out, a manual that is still contained in its original shrink-wrapper is not a good sign.
Could be a sign of problems…
Or of a mind that for whatever reason cannot remember how many quarts of oil the vehicle is supposed to need for an oil change.
Why I can NEVER seem to remember that little tidbit of info, but can tell you scores of football games I went to 30 years ago, I’ll never know… 
Eraser, Do What Some Guys Do And Paint It Onto The Top Of The Engine Or Radiator Support.
I keep spiral bound log books in all of our too many cars and start my oil changes there, recording mileage and date. The log book also has written in it the amount of oil, type / brand of oil, filter number, and drain plug wrench size. I also log any repairs / maintenance, miles, and dates.
All of our cars get oil changes when the odometer ends with 5000 or 0000 (5,000 mile increments), give or take a hundred miles or so. That’s one thing I don’t have to try and remember or look up.
CSA
They have maintenance schedules - it’s just integrated into the car now. So instead of trusting the driver to read the manual and check the schedule, which most never do, now a big bright message comes up in the instrument cluster, complete with a ding-sound, to tell you to go have maintenance done.
Not a bad suggestion with the paint - I’d never thought of that. Somehow I never seem to have trouble remembering WHEN to change the fluids - its just how much capacity each car takes that I struggle with. And those Fumoto valves make oil changes so easy it takes longer to think about doing them than it takes to do them…