If you can look over before buying
Check the fit of the ferrules and make sure they are not to loose. Look for wear or rust on the guides. Run your hand down the rod and feel for rough spots. Test cast it if possible, you might want to bring your own reel and fly line to fit the rod weight. Do not worry about the cork being discolored and do not believe a clean cork handle means its had little use. Its to easy to clean up the cork.

On line.
Ask questions or the seller about the rod. Do make sure the seller is well represented at the place he or she is selling. If possible set a couple day return option if it does not meet the sellers description to be returned. I have found so far all of mine have been better then described. Get a rod from a good quality maker.

I have bought all but one of my current rods used. All were at least half off of retail. Some were old glass rods and some still had the plastic on the grip and all the tags still on the rod. Only trouble I have ever had was one that the ferrules were loose on one bought on line and it broke after a half year of use. But it was from a name brand company who replaced it for a small fee since it had come with a warranty card with it that the original user still had after the many years he used it.