A picture is worth a thousand word.
If you are going to sell your used lens, the first thing your potential buyers will see is its exterior. The optical condition is the second.
Many buyers usually browse through the listings to find the items they want. An eye-catching photo showing a mint lens can definitely stand out from the rest. If your item cannot catch the buyer's attention in the browsing, you may miss the customer.
For many lenses, the exterior rubber rings (focus and zoom) are prone to oxidation and turning white after some use. Even the function and the optical performance of the lens are still very good, the white rubber rings will give an impression that the lens has been heavily used and is inferior to others.
After you have cleaned the exterior, you should check and clean the glass. Pay attention that not all lens glasses are really glasses. Some are made of much softer crystals or minerals. They could not stand even the slightest wipe by a microfiber cloth. I recommend you use a blower to blow away any dust on the surface. If you really need to wipe, wipe it carefully and on the right spot only.
You should know your lens well. Your buyers may want to get further information from you about the lens. A timely, precise and concise reply can definitely make the buyer feel that he is buying from a knowledgeable seller.
If you aware of any issue of the the lens, be honest and list them in the description.
The original packing material can definitely give a boost to your reselling price. Buyers prefer complete set of product.