I have bought and sold several lenses on Craigslist. The advice about meeting in public is a good one. A Starbucks or equivalent is a good option. Ask questions via email before meeting up -- if the person is non-responsive or cagey, walk away from it. If you are buying a lens, bring your camera(s) and test the lens before pulling the trigger. If you are buying a camera, bring a lens or two that you'd likely shoot with.
On the plus side, I have gotten some great deals. I bought a very clean TS-E 24 II from a former pro for $1,300. The same guy emailed me two months later and asked if I wanted to buy his 135L for $500. It pays to develop a rapport with other gear heads.
Ebay is tougher because you cannot test the lens or camera before buying. I bought my TS-E 17, 100-400, 100 macro, and 50/1.4 on ebay. The TS-E 17, 100-400, and 100 macro were in very good condition, but the 50/1.4 was in poorer condition than stated in the ad. If they include a filter with the lens, it will likely be garbage so do not factor in that cost to the total amount that you would be willing to pay.
Before you buy any used lens, know the new cost, refurbished cost, and going prices on ebay and Amazon. Set a limit and do not vary from it. I have had no problems with my used lenses.
If you are selling, if you are willing to take a slight hit on the price, do so. You will sell your gear twice as fast. With my TS-E 24 and 135L, the guy was willing to part with the lenses at very favorable prices. All I had to do was get there faster with the cash than someone else.
The flip side to all of this is buying new. I bought my 24-105 for $1,150, my 70-200 f/2.8 II for $2,500, my 24L II for $1,650, etc. I have had the opportunity to buy all of them for a lot cheaper since my purchases, which bugs me a lot. So used gear can be a good thing -- the key is getting a good deal on a clean copy.