The following is an excerpt from Kardec's comments in Chapter 26 of "The Gospel According To Spiritism". 

FREE MEDIUMSHIP

Like the apostles who were mediums in their own right, the mediums of today receive their gift freely from God.  The gift consists in the ability to serve as an interpreter to the Spirits who come to impart teaching, to lead people to the pathway of goodness, and to nourish their faith.  It isn't the result of their own ideas or research or personal work.  Note here that God wants the light to reach everyone- not to disenfranchise the poorest of the poor.  The poor should never be able to say that they don't have faith because they couldn't pay for it, or that they were denied encouragement and expressions of affection from their departed loved ones because of a lack of money.  For this reason, mediumship isn't limited to a privileged few- it is found everywhere.  To make someone pay for it is to turn it away from its providential purpose.

People who understand the conditions under which good Spirits communicate, who recognize the disgust these Spirits feel when confronted by anything that smacks of self-interest, and who know how little it takes to drive these Spirits away, can never accept the idea that Superior Spirits are at the beck and call of the first person who comes along and - at so much much per session- claims contact with them.  Simple good sense rejects this notion.  Isn't it an act of disrespect to call up, for money, highly regarded Spirits, or our loved ones?  Now, there's no question that a medium can receive communications in this way.  But who can really vouch for the genuineness of such messages?  Thoughtless, deceitful, mocking Spirits- the whole crowd of unevolved and unprincipled ones- always come running, ready to answer any question without regard to the truthfulness of their views.  If you want serious answers, begin by posing serious questions. You should also consider the nature of the affinity the medium has with those in the Spirit realm.  The conditions for enjoying the goodwill of noble Spirits are humbleness of heart, devotion, selflessness, and total disinterest, both moral and material.

Besides the ethical question, a second consideration, no less important, comes to the mind: the nature of the gift itself.  Serious mediumship is not, and never can be, made into a profession.  First, from the ethical standpoint, the idea would carry little credibility with people, and second, it would be quickly seen as akin to mere fortune-telling.  But there is another obstacle working against it.  As a gift, mediumship is basically unstable, elusive, changeable.  No one can count on it as something permanent.  As an exploitable resource, it is highly uncertain and can fail when most needed.  In this respect it differs from a talent acquired through study and work which, because it is the product of one's own efforts, can rightfully be used to one's benefit.  Mediumship isn't an art or skill, and no one should consider it as a professional activity.  It exists only to the extent that the Spirits are willing to cooperate with the medium.  If the former aren't present, neither is the mediumship.  The potential may still exist, but not the practice.  There isn't a medium in the world who can guarantee that he or she will be able to produce a Spiritual phenomena at any given moment.

To exploit mediumship for gain is to rely on something over which one doesn't really have control; to claim the contrary is to deceive the person who's paying for the sitting.  What's more, one is not really relying on oneself, but rather on the Spirits, the souls of those who have departed the Earth and on whose assistance one has put a price.  The idea causes instinctive disgust.  It was this same kind of trafficking and exploitation by charlatans- along with the ignorance, gullibility, and superstition of the people- that moved Moses to ban communication with the dead.  The Spiritist Doctrine, through its understanding of the serious nature of the question, has completely discredited such exploitation and elevated mediumship to the category of a mission.

Mediumship is sacred and should be practiced in a saintly and devout manner.  If there's one type of mediumship that demands this attitude more absolutely than the others, it is healing.  In the medical world, a physician offers his or her knowledge, often gained at the cost of painful sacrifice.  A healing practitioner gives of his or her own energies and sometimes even health.  A price can be put on their services.  The spiritual-healing medium, however, relays healing energies from the good Spirits themselves and so doesn't have the right to charge for them.  Remember that Jesus and His apostles, although poor, didn't charge for the cures that were brought about through them.

The Gospel According To Spiritism Ch 26, Freely You Have Received, Freely Give,  "Free Mediumship" pgs 252-254