All good advice here. But also remember, a used bike is just that - a used bike. Some of the signs of abuse, can also just be signs of age. Especially with really old bikes.
My 1990 ZX-10 has some of these signs. But not from neglect, just from wear and tear of 17 years 35,000 of riding. There are some cracks in the fairings, scuffs on the bar ends, and a bunch of other miscelanious stuff. But you shouldn't see these same thngs on a bike thats only 3 or 4 years old. So take the age of the bike into consideration when looking it over. And some things, like badly worn sprockets, corded tires, etc. are unexcusable for any bike you would buy. No matter how old it is.
Your best weapon is to actually take 20 minutes to talk to the seller. Ask good questions, that he can't just answer yes or no to. Things like "how long have you owned the bike" and "what's your maintnance schedule like?" Just like doing a job interview, you want him to open up and actually talk to you about the bike. You can tell alot by how someone talks. Ussually, it's pretty obvious if someone is trying to get over on you if you "interview" them. If it feels strange when the owner can't tell you about that crack in the fairing, and he's owned the bike for 5 years, it probably is strange!
Someone who has only had the bike for a year, may not know a whole lot if he bough it used as well. But I personally can tell you about almost every imperfection on my bike. About the crack in the fairing where the mirror mounts to it, from parking it in the soft grass and forgetting to put the plate under the kickstand. Or the scratches on the left side of the fuel tank, where my pocket-knife's clip rubbed when pushing the bike around the garage. The broken belly-pan fairing from running over a raccoon in the middle of the night.
If the owner has stories for these little these little nicks and dings, and can talk to you at length about how they happened, what he had to fix, and how he was so frustrated at himself for doing something so dumb, then it may be a great bike that is just a little old and showing some personality. But if he can't tell you about it, and just kinda skirts the issue, good chance he's trying to get over on you.
Almost every bike I've ever owned has been used. Some, though they looked a little worn, were lovingly maintained. They were just "well-worn". But it also gives you some negotiating power on the final purchase price.
If you can, take someone with you, that you know and trust, who knows about bikes. They can help look over the bike, and tell you what are potentially major problems, and what is normal wear and tear.