Having been in professional sales for over 30 years I would have to say it is a combination of the two. You have to be born or at least developed at a young age to become the kind of person who lives to help others. After all sales comes down to finding a need or problem and solving it in the most effective way.
From here it becomes a process of developing and honing skills and knowledge base to make the most effective use of every possible resource.
When it comes to a mediocre sales persons potential I think it really comes down to assessing their personal drive and passion to help others. If that spark is present then the potential is there to develop the skills and knowledge base to be a top seller.