With some exceptions, women who opt for the work of a prostitute are usually young, uneducated and in need of money for substances that they should not be consuming - drugs, tobacco and alcohol. They are easily led into the life and because they do not continue their education, find no way out. When they are too old to be of any use to men who want to buy a prostitute, they don't fare very well. They also have low self esteem because they don't have any other type of skill or work to support themselves and feel trapped in their life on the streets and hopeless of any release from their bondage.
Prostitution is psychologically harmful to the individuals doing the soliciting because it devalues and demeans them as human beings. They are routinely treated as only a collection of body parts with no real, intrinsic value. Their needs to be important and valued as a person are subjugated to the John's needs for physical release.
It's only harmful if society still dehumanize prostitutes. Once society can fully accept prostitution as a normal job and there are regulations about it so that the client can't abuse them, prostitutes can take pride in their job and no longer feel that they are less than a human.
Prostitution is just like any other job, you are selling (or rather giving service) using your own body. It's consensual sex between two conscious adults in a private space and harms nobody, so society should not degrade the job of a prostitute.