What does it mean by "Selling one’s soul to Devil"? I became very interested (by it means, curious) about the selling soul to devil in past few weeks? You probably had heard the most popular example of this: Robert Johnson, Paganini etc.  I researched alot about this and someone tells the method to do,  someone said it can be done in pieces and one question I found on Stackexchange and answer says that it cannot be done at all as we are not owner of our soul but God himself.
So what's this actually means? And if somehow somebody able to did this,  what consequences he has to suffer? How could the myth of Robert Johnson or Paganini be explained? If selling the soul stuff is absurd then why does this concept even exists?
PS: I am not Christian (so sorry if I wrote something wrong)  but believe in God and the spiritual war with the Devil.
'Selling one's soul to the Devil' is an idiomatic English expression but it does not occur in the bible.

Jesus says :

what shall it profit a man, if he shall gain the whole world, and lose his own soul? [Mark 8:36 KJV]

That is to say, in pursuing worldly things, one neglects the salvation of one's own soul.
James the apostle says :

Submit yourselves therefore to God. Resist the devil, and he will flee from you. [James 4:7 KJV]

He urges submission to the mighty hand of God and to resist devilish temptation.
Of Judas, who betrayed Jesus, it is written :

And after the sop Satan entered into him [John 13:27 KJV]

After Jesus passed him some bread for dipping, Satan entered into Judas and he went out and betrayed Jesus.
But I am not aware of any biblical text that would support the idea of a pact being made between a human spirit and the spiritual Entity called Diabolos, Satan and Dragon and Serpent, regarding the soul.

Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.  [Ezekiel 18:4 KJV]

Souls do not belong to the Devil, nor can he be negotiated with in regard to them. That is a superstition and not a fact.
Souls belong to God, as Ezekiel tells us. And God, alone, is the possessor of them. And God alone shall recall them, at a time of his own choosing. And God alone shall judge them.
For he, alone, is God.