For me, buying used lenses is a very picky time for me. What I am looking for is "new" condition at a decent savings over new (If still available). If I can't personally examine the lens,
ebay, for example, there has to be lots of pictures, and a highly trusted seller. The pictures need to show good closeups of the lens, especially the front and rear elements, and the electrical contacts. If the contacts are well-worn, the lens has lots of miles on it.
Presuming I want to buy that particular lens, I'm looking to save at least 20% over retail. Back in January, I was looking to get a Canon EF 24-70 f2.8L, and found that 'barely used' copies were going for about $100 less than full retail. I decided to buy new and get the full warranty.
I did buy a used Canon EF 200 f2.8L II at about 25% off full retail - mint condition, of course. I also bought an out-of-production Canon EF 80-200 f2.8L "black pipe", mint condition, for 1/3 of what a new 70-200 f2.8L II IS runs. The savings there made it very worthwhile.
As mentioned above, check out the 'going prices' for the specific lens you are interested in, and also check out the prices or better quality comparable lenses from other manufacturers. It's real easy to decide "this is the lens I want" and pull the trigger, only to find out a couple weeks later you paid more than you needed to. Bottom line...WAIT for the deal you want on the lens you want. There's usually no need to get it this week, vs, a month from now.