Unlike most articles of clothing, shoes cannot be altered or cleaned. Also unlike most articles of clothing, the uppers of shoes take on creases and bends depending on the size and shape of the original wearer's feet and his or her gait. There is also stretching of the uppers depending on the size and shape of the foot. These factors can combine to create a loose fit in which the foot of the second wearer is not stabilized, cause undue friction for the new wearer, or both.
Heel and sole wear also varies with the wearer and, depending on the material, the insoles may permanently conform to the shape of the wearer's soles. The wear pattern may put stress on a new wearer's feet and/or legs. Is saving a few dollars by wearing used shoes worth the possibility of injuring your feet? Only you can make that decision.
There is one exception to my suggestion that you not buy used shoes. Many people have shoes they seldom, if ever, wear that they received as gifts or bought for special occasions. Assuming they have no significant creases or wear, such used shoes can be a very good buy.
