Many will tell you that selling your soul is not actually possible.
You also should consider why an entity might want to claim as much, according to somem other beliefs.
People who do believe in entities offering to trade souls for favors will tell you that not only are they more often lesser beings craving energy, but that the tradeoff is almost always not woth it. And promised offers are frequently lies.
if you already believe in reincarnation and the eternity of the soul, then you believe you are already immortal in the only feasibly real sense.
If you are practicing magic, you are already developing and exercising powers.
A malicious entity, or even just a trickster, may play such word games to get an advantage in a trade.
Consider, in stead, calling for aid and participation of specific entities in your workings.
If you follow such belief and a path incorporating spirits of some sort, you will eventually get to know some. Develop a working relationship. It will be much more rewarding.