You raise an interesting question.
I was tempted to say "all business need customers" and think that was all there was leave it at that there was to it, but it's not quite true.
* Your example -- traders trading their own portfolio.
* Tasks that are open to all and paid by result -- the mechanical turk, various affiliate marketing programs
* Mining crypto currency
* Posted challenges like the X-prize
etc.
That said, two thoughts for you.
1. Successful business are rarely run by single person with all the skills. You can potentially get a partner or employee that is good at sales
2. Learning sales early in your career is a really good idea.
This Coursera specialization is a good place to start
https://www.coursera.org/specializations/the-art-of-sales-mastering-the-selling-process