How could I be a great sale man?

Yes, of course.
Anyone could be a salesman.
Remember, leaders are not born.
They are made.
If a person is professionally trained in the field of selling, and he/she continues to get exposure to the knowledge and good reading he/she could become a good sales person.

There is no exact yes but the integrated people with communication and tacit knowledge can be good sales persons..

not any one can be a sale man because sale man is a very tough job that needs a lot of effort and commitment, trust and a true promise.
The customers depends on them in knowledge wise and as well as brand choices its needs a heart to do it many think they can be sales persons but they do not fit in.
every one was born with his or her own talent in life.
just like not every one can be manager, some jobs needs experiences while some needs skills and talent to do so