Even though it should go without saying, I’m going to say it. Do your homework! It’s not enough to know everything about WHAT you want. You may have read all about the sweet spot on the Nikkor 24-70 f/2.8, or the minimum focus distance of the Canon 70-200 f/2.8 L-Series, but when you’re getting ready to drop a significant chunk of your hard-earned money on a lens, you need to find out everything you can about the person selling it.
If you’re contemplating a face-to-face transaction, ask around. Get references or referrals if you can. In this day and age of Facebook, Twitter, and Google+, chances are good that buyer and seller each know someone in common. If you’re buying on ebay, carefully check the seller’s feedback and ratings, particularly the comments. Also take a few minutes to compare the price against other auction sites and even that of a new lens.