andrea tasselli:
I still have a Nikkor lens (300mm f/2.8) with a small chip missing fron the front element. Can't see it affected the images in any way. Same with fungus. One of my sharpest lens is heavily covered in such in one of the internal lens. Same story. So it depends on what the defect is like.
P.S.: A scratch would have no effect at all.
Andrea, that's what I meant with "How bad it is you will only see after trying it out". In my understanding "a mark" is more severe than just "a scratch". You can be lucky and get away with it or it could be totally unusable. Another point in my opinion is that the mark is an indication of rough handling of the scope which may lead to all sorts of aftereffects. The seller could be an idiot who sells a perfectly usable lens for a way too low price or he could seek for an idiot who buys a unusable lens for 300 EUR. Most likely the truth is somewhere in between but I wouldn't take the chance. Obviously the risk assessment may vary.
Clear skies
Wolfgang
