With all the incredible deals available on ebay, and the prices of new running shoes, many people will be tempted to buy used shoes.  Here are some things you need to know:

• If you are a new runner, your first task is to build enough muscle to control your joints.  Don't expect the cushioning in your shoes to protect you from lateral motions of your knees or ankles. Uncontrolled lateral motion will cause pain and injury.  The lifespan of your running career is mainly limited by the condition of your joints.
• If you have flat feet, get orthotics from a competent sports podiatrist before you start training.
• Almost all of the cushioning is in the midsole, between the outer sole and the insole.  The midsole is stiffer, often polyurethane, for heavy runners.  The midsole is made of EVA for light runners.  EVA is basically high grade foam rubber.  Magnified, it would look like a sponge.  The walls of each bubble stretch to absorb impact.  Some manufacturers use air, gel or liquid capsules in the impact areas; these work on the same principal.  Saucony used a grid, which works sort of like a tennis racquet.
• When repeated impact drives the air out of the tiny bubbles in the midsole, the shoe no longer protects your joints from impact.  One indicator of a worn midsole is "compression set".  The midline of the heel cup should be perpendicular when the shoe is resting on a flat level surface.  If the midline is tilted, even a few degrees, one edge of the midsole is more worn than the other.  The reason this indicator is useful is because most people heel strike on the outer edge, not in the center of their heels.
• Rule of thumb: if the outer soles look worn, the midsoles are always worse.
• If the outer soles look new, that does not mean that the shoes are like new.  Some people run on indoor tracks or treadmills, which preserves the outer sole but not the midsole.

New topic: fit.

• Running shoes should feel comfortable from the first time you put them on.  They do not require any "break-in" period at all.  Don't use thick socks to make yourself tolerate an uncomfortable shoe.
• Size does not guarantee fit.  The same size from different manufacturers (and even different models) is often significantly larger or smaller than your street shoe size.  If the position of the arch does not match your foot, they don't fit you.  If the widest part of your foot does not line up with the widest part of the shoe, they don't fit you.  The distance between your longest toe and the end of the shoe should be about the width of your thumb.  Really.

Finally, remember that you can't get 400 miles from a shoe that has already gone 200.  Avoid brand X, because the quality control is often unsatisfactory.  But, don't think expensive shoes will always last longer than mid-priced shoes.  (It's not like buying a Lexus.)  For some people, it is far more sensible to buy less expensive shoes and replace them often.

By the way, if you're a new runner, be patient with yourself.  Smell the roses.  Learn good form.  Build yourself up to the bigger challenges.  When training, trade speed for distance.  Don't forget the water.  You can run through fatigue, but always stop if you have a sharp pain or feel dizzy.