I've never bought a used bike. I am interested to know your sorry experience in buying one, e.g. what kind of problems you had that the seller did not disclose to you and you did not know until after you bought it? Did it cost you a lot to fix the problem?
Although used bikes are cheap, you don't get free service from LBS as you would when buying a new one from them, so this is one thing to think about for someone not good at adjusting/fixing bikes.