Purchasing a used lens can be a little trickier than buying a used camera because it's not as easy to check its functioning. The key is to test your photographs before you make the purchase. 

Here are some tips for buying a used lens:

1. We like to start with a careful inspection of the lens body, including the front and rear elements. Check for dents (could mean it was dropped and damaged) or scratches (some are normal; a lot means heavy wear and tear), wearing-off of lettering and numbers, and, lastly, check for any loose parts including screws. If you’re looking at a lot of obvious wear and tear for a lens being sold as “almost new” you know that’s not correct. 
2. Carefully check the reflection on the glass looking for any unusual marks on the lens’ coating. 
3. How much dust or fungus do you see when you hold up the lens to a light source? You’ll have to decide how much is too much of either one for your comfort level. For some photographers, any evidence of fungus is a deal breaker while heavy dust likely indicates a lens which hasn’t been well maintained.
4. Mount the lens to the camera to ensure there aren’t any issues with mounting or when you slightly press down on the shutter. There should be no error messages.
5. Check for auto-focus when attached to the camera. Any issues? Also, check for focus throughout its entire zoom range.
6. Take a picture on AF and check to be sure the focus works. 
7. Make sure that the focus scale matches what the lens is doing and that there aren’t any cracks. 
8. Try shooting on manual focus to ensure there aren't glitches with either the zoom or the focus. Check to be sure that any manual overrides work as they need to when you want them to.
9. Next, check the filter threads for any dents or flat spots. Bring along a filter to mount to be sure there are no problems.
10. Check all of the lens’ features to be sure they work including IS.

The safest bet is to do a thorough inspection of any used camera equipment before you pay and walk away with it. Remember to carefully test all of the equipment by shooting on various modes and focus. Read up on what to expect from the camera and lens before you purchase it and ask for the original paperwork including receipts. Those could come in handy if you need to service the camera.