Well Elanamig,
If you must buy used, try to at least join a bicycle co-op, first. Volunteer for a few days and try to make friends. The co-op might even have a donated bicycle there that you can build up for your own. If not, that's alright too. You can always have the used bike seller meet you there at the co-op. Have one of the mechanics there evaluate the bike, before you make the purchase, and after you've test ridden it.
Old steel framed bikes are practically fool-proof, if they fit. As long as your steel framed bike fits and most of the components are in good condition, the bike should make for a good buy. Watch for dents, gouges, cracks, and rust. If the components show signs of rust, chances are, that the external part of the frame has been cleaned up, but inside the tubes are partially oxidized. It doesn't matter about the age of a steel frame. Therefore, the year won't matter. Component replacements will be easier to find, if your steel bike is from the 90's or 2000's.
Avoid used carbon at all costs. Be careful about buying old used aluminum bikes. If the aluminum frame looks new (without any scratches and not repainted), the components look good, and the bike performs well, you should most probably be willing to risk it, as long as it fits.
Last edited by Cfiber; 05-15-13 at 09:31 AM.