After many decades of research around the factors that make a great salesperson scientist and psychologists still don't agree on the extent to which the success of a sales person can be related to their genes and personality or their effort and dedication.
Like everyone I sometimes have to deal with a salesperson that is particularly good and manages to make me buy a product or make me spend more than my initial budget. I would say that this is related to his/her natural selling talent. These people know their product like no one else so they make sure that you hear the strong points and they try to skip the negative ones.
I would have to agree that some salespeople seem to be more naturally suited to the job than others.This can be due to their genes, personality, social interactions. This group of people are usually better at building relationships and more able to get their message across.
(source: http://www.theallusionist.ca/tag/malcolm-gladwell/)
On the other side there are theories like the 10000 hour theory developed by Malcolm Gladwell that could explain why some people still think that everyone can be trained to do a certain level. According to this theory it takes roughly ten thousand hours of practice to achieve mastery in a field.
To conclude, training a sales person for 10000 hours seems pretty unlikely and expensive so I think that a great sales person needs to have a certain balance between training and natural skills or be extremely talented as training for a low number of hours doesn't compensate for the lack of natural skills.
http://www.wisdomgroup.com/blog/10000-hours-of-practice/
http://www.gaviningham.com/are-good-salespeople-born-or-made/
