Many things could have happened to a used lens before it met you.
It could have been dismantled, repaired and cleaned many times over the years.
You can observe the scratches on the screws and wipe marks on the glasses to know part of a lens's history.
You can also rattle it gently and hear if it has any loose parts.
How about smelling a lens? What do you expect to find out?
Well, a proper lens lubricant don't give a strong and distinctive smell. If you smell WD40 or other volatile lubricants and solvent like the lighter fluid, that means the lens has recently been lubed by them. The volatile lubricants usually evaporate in a few hours to a few days. If you can smell it, it must have been applied recently. That means after the volatile lubricant evaporated, the lens may not be focus as smooth.
But why would someone lube a lens by WD40 and lighter fluid? Because it's a quick fix. One can lube a lens by WD40 and lighter fluid without taking the lens apart. The thin and volatile lubricant can easily seep into the focusing threads and soften the aged grease. The focusing could temporarily become "butter smooth" again. Yes, the old and sticky grease is thinned and softened by the lubricant and becomes "butter". After the volatile lubricant slowly evaporates in a few days, the "butter" becomes hard and sticky again. The focusing would become tough or even stuck.
If you search "lens rubber grip whitened", you may find people showing "creative" and amateur ways to restore the whitened rubber grip, such as using WD40 (again!?), rubbing alcohol, hand soap and even olive oil and vegetable oil. None of them is doing any good to the lens rubber grip. The organic solvent like WD40 and alcohol would etch the rubber surface and make it prone to further deterioration. Some may even attract bacteria and cockroaches to have a bite of the rubber.
You may want to see how our Lens Rubber Cleaner can rejuvenate the lens rubber grip beautifully.