Marks on a lens can be caused by scratches, or by improper cleaning. “Cleaning marks” are caused by improper cleaning. If you wipe a lens with, say a dry Kleenex, then you can scratch the lens coatings (not the glass, but the coatings on the glass, which are softer) especially if there is something hard, like a grain of sand, on or near the lens. Most used lenses will have some very light marks if you look with a bright flashlight at an angle. Such cleaning marks will have no effect on image quality, except, perhaps, if there are lots of them and you are shooting straight into bright light, where you might get more glare than otherwise (though it would be very hard to tell). Even serious scratches on the front of the lens will not show up at all in the pictures; they become entirely blurred out during focusing. Google it, there are some amazing sample shots of images through cracked lens filters, and you can’t even tell!