You don't come to his(her) door by accident.
There are no walk in visites. You know someone and they make an appointment for you.
When an appointment is made you're given directions, and (s)he always just so happens to live close to you, you'll come to a purple door. Maybe the door whats always there, maybe it will be gone tomorrow. Maybe the door is in a small building or maybe it's in the back of a restaurant. The door is always ice cold to the touch. When you open the door you are greeted to room that is as (un)pleasant as you want it to be. (S)he is always sitting in a purple throne.
When the Soul Trader talks, and (s)he will move you with speech after speech, you will feel compeled to take his(her) offer. You'll know full well the consequences you may have even dreamt of them the night before, but you will, sure enough, take the deal.
This deal will, at the time, seem like a bargain. The feeling of warmth as you shake his(her) hand will stay with you as you leave the room. Whatever your terms were will be fulfilled to the very last letter. Your eyes will be filled with most golden of suns and the moon will shine it's heavy rainbow halo's on your bed as you sleep. But, soon. Not the first year, or the second, or maybe not the first ten years, but one day, and you will know the day. The Soul Trader will find you. Every door you see will be the same etherial purple. Every road will lead in circles. You will be called, you will come back to his(her) room and you will fulfill your end of the bargain.
Unless you can recommend someone more substancial.