When I've been interested in used lenses online and not being able to go and touch and see, I specifically as the seller to confirm that the glass is scratch, fungus, fog, dust and any other blemish free, and that the body is an equally good condition, he zoom smooth, no sticky points etc.
This will help protect you should it not be as described then.
Also ask for unedited, pictures from various focal legths on the lens you're after, and ask for specific items to be photographed.
Again, this helps prove that the lens is in good condition, and that the seller is not just showing you pictures from yesteryear when the lens was in its prime