Nobody can sell you a spirit. On the “food chain” of beings, humans are granted certain abilities by Nature, but the ability to buy or sell an alterdimensional being is not one of them.
Some conjurers may use the term “selling” as a sort of a short-hand for “smoothing the way for you to have access to…” but they are not actually selling you a spirit. If they are not totally scamming you, and they actually do the work, they are just weaving a channel between you and that spirit for communication or flow of a certain energy.
An Orisha is a force of Nature. Nobody can sell you a force of Nature. They can initiate you into the cult of that force of Nature, or make a way to allow you limited access to a degree of energy from that force of Nature as is appropriate for a non initiate or layperson, but they can’t sell you the Orisha.
Djinn, gnomes, ndoki, faeries, elementals, and other alter-dimensional creatures can be enlisted or employed, but still not actually sold. They will allow themselves to be utilized to a degree, so long as a human goes through the proper steps, but one must be very careful.
There is a sort of image that some conjurers choose to present to the public as one who is able to command demons and control djinn and such, and this is legitimate since it works. However, one should realize that this image is allowed by the spirits only because it is useful to convince some people who wouldn’t otherwise, to accept help. Many entities are fed by happiness and success, and for them it doesn’t matter so much if the humans think it is because of the powers of the conjurer or the powers of their own or of Nature. They are just happy to help, and if it means playing the “genie”, then so be it. It takes nothing from them.
However, it is extremely important to remember that these are extremely powerful whole beings. They are not actually our slaves or servants. Like a polite, very helpful, or heroic human may choose to work as a nurse, a soldier, or go into some other field of service, this does not mean they are weak or obligated. They choose to be there for us or not be there for us. They choose whether or not to help us or to leave us, or to destroy us.
If you disrespect the spirits who help you, they will abandon or destroy you. Most of the time, you are given an nkisi or another item such as a stone or ring that is a sort of anchor and bridge between that being and yourself. These must be treated as sacred items. Some have more endurance than others, and can be worn or carried in a pocket, but they should still be treated with respect.
If your helper requires offerings, those should be kept up to date to the best of your abilities. You need to follow the instructions you were given very carefully, and make provisions for the case that something may happen to you, and they need to be released.
…and this is just if you got an actual properly enlisted djinn, ndoki, faery, or other being. Quite often, especially if the conjuring and maintenance was “easy”, what you may get is a mischievous or predatory, often called a dark entity that feeds on misery and unhappiness.
This is not to say that all of the positive ones are difficult to conjure. It’s just that a big part of the “summoning” ritual is to filter out harmful entities or imposters. Most ceremonial magicians take weeks or months to call certain entities, and still don’t always succeed. As Brother Rahman explains in the link above, we can do a long stint of protective prayers, fasting, filtering, calling, negotiating, “commanding”, offerings, and more, and in the end…nothing. Sometimes no spirit wants to come to assist for a particular goal. We just have to accept that.
…but this should tell you, they choose, not us.