The first thing to do is to, of course, get the seller to agree to "lend" you the lens for your own testing. This should be no longer than 7 to 10, days depending upon how long it takes to process the film and make prints. If this is not possible, then make sure you can return the lens for a full refund during a certain time period. Most reliable retail establishments have a 15 to 30 day return policy.
Scratched lenses can result in flare which of course reduces image quality.Â If you really have to have the lens, the fix may be to touch up the scratch with flat black paint using a very fine brush or marker. Â Attempt this at your own risk and only if you have the hand of a brain surgeon. This is only necessary if the scratches are heavy and on the front element.Â Small scratches, especially those on the back element of the lens, won't make much difference.Â Scratches on the sides of the elements are also less likely to create a problem, especially when you stop down it may not even be in the light path.
Fogging or haze...The best way to check for fogging or haze in a lens is to shine a bright flashlight through one end of the lens and look through the other end of the lens.Â The fogging/haze will show as a white colored cloudiness on the lens elements. Sharpness and contrast suffer from haze because of an increased tendency for the lens to flare. In most cases it can be cleaned professionally. Some cases are so severe because of deterioration of the lens coatings.Â If this is the case, then the lens cannot be cleaned properly without removing the lens coating. This IMHO is not a good thing though there are those who might disagree.
Fungus problems occur alot in humid climates. Living near large bodies of water or having d& basements are the usual culpritsÂ Fungus can actually etch glass. It sometimes can be cleaned if it is detected early. Looking through the lens you may see faint white lines/marks that either look like frozen lightening or jagged concentric circles. Then there is always the nose test. Take a wiff, if there's mold present you may smell it. This latter test was a prime way to tell if the camera had fungus damage inside.
Coating marks are often the same as cleaning marks.Â You see these by holding the lens surface perpendicular to your line of sight. The soft flourite coatings of Leica lenses are easily scratched by improper cleaning.Â A small amount of lens cleaning marks or coating marks may make no perceptual difference in the performance of the lens. A large amount of cleaning marks or coating marks will make a small difference by possibly introducing a little flare.
BTW, if the lens is of great value, you probably know to get yourself a second or third opinion from someone whose judgement you trust. You'll never know what they'll find because they are not in a "must have or die" mind frame.
I hope this helps a bit.
Good luck in your search