Is it possible to sell your soul to a person?

Yes, the person you are getting the soul from has to willingly give it to you, have the person you are getting the soul from write on a piece of paper, "I <insert their name here> willingly give my soul for use in any rite, ritual, or barter to <insert your name here>. <their signature/some of their blood>"

Now you own their soul, if you wish to sell it to lucifer, mark a pentagram, place the piece of paper in the center of the pentagram and light it on fire, say "I will sell you this soul(s) in exchange for <what you want>." Then put out the fire with boiling unholy water, if the paper stops burning/turns to ash, the devil has rejected you barter.

Warning: the pentagram is a gate to hell you have opened with the boiling unholy water, remove the pentagram immediately after your transaction with holy water.