I think for any second hand equipment. It's very hard to put a 100% accurate
definition on these so called "condition rating", I don't know about other
equipment, but for lens, you will need to ask flat out for the following
areas if you don't see any real photos of the actual lens.
1. lens glass(elements). Ask the seller, if there is any visible defect such
as but not limited to scratches,crack,dust,marks,fungus, separations.If
they claim they are not professional and they cannot see. That's possible BS
since you don't need professional experience to just look. If you want to
risk it, ask him to open up the aperture and take clear photos of lens with
strong/light in the background and send you the photo for you to see. If he
refuse for any excuse. avoid.
2. mechanical operation. For older lens you can ask if the turning of
aperture ring and focus ring turns with problems such as stiffness, play,
slip or uneasiness.
For newer lens it's a lot harder to determines, so go for those they claimed
to have been tested with appropriated cameras. but limited test such as
focus ring is still testable without camera.
3. exterior condition. Ask if any dent, crack, modification, lost of paint,
marking, missing parts, dirt buildup. etc.
4. What is included in the sale, front/end cap. manual, box, etc.
If they said no on anything but the lens come with any or those defect, you'
ll have proof. If they give you any excuse like they are not professional
and don't know what to tell you. walk away.