Thanks to Adam for organising, and also fatigue for the helpful tips: Took notes .. intend to post it later.
But here's a useful link that recaps most of what fatigue talked about
http://www.petapixel.com/2011/09/27/how-to-detect-physical-flaws-in-a-used-camera-lens/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+PetaPixel+(PetaPixel)
If a camera lens has been abused, mishandled, or is just plain worn out, there are telltale signs that a knowledgeable buyer can look for to help appraise the value or lack thereof in a used camera lens.
Glass
The glass of a lens can have numerous problems. Most easily detected are scratches on the exposed glass elements. Sometimes, lighter scratches caused by poor cleaning technique can be present and are more difficult to detect. Bright reflected light is usually sufficient to see “cleaning marks”. Examine the lens with the light reflecting off surfaces at several angles and you should be able to tell if any light scratches are present.
Blemishes
Lens elements are generally “multicoated” with layers of nonreflective optical material. This minimizes light reflection and the resulting lens flare and ghosting associated with the multiple reflective surfaces of these complex optical devices. “Blemishes”, areas in the multicoating where material has been smeared or removed by a bump to the glass, manufacture defect or a solvent splash are less critical flaws provided they are very small (< 1mm diameter) and few in number. A small blemish shouldn't affect image reproduction. .....
p.s. I was the chap who registered via SAM and was seated at the front right corner, rofl was asking fatigue if the seat is taken and I didn't know he's the speaker!
