Willy's natural talents lie in working with his hands. He tells Charley, "A man who can't handle tools is not a man," but Willy doesn't value this kind of work. He even says his father was better than a carpenter because he made flutes. Willy can put up a ceiling and build a porch. Biff recognizes that both he and his father should be working with their hands.
When Willy thinks, however, that one of the salesmen in a store refers to him as a "walrus," he strikes the man. That kind of behavior isn't appropriate for a man who is trying to sell products to a business. Willy's personality is too volatile and unstable to succeed as a salesman; he has, furthermore, unreal expectations for his efforts. When he feels discouraged, he seeks the company of The Woman, who makes him feel special and popular. He can ignore his lack of success until he comes home to Linda and reality. Although he is persistent as a salesman must be, he lacks the skills and personality essential to a successful salesman.
A good indicator is Willy's personality and tendencies. Willy simply wasn't made out to be a salesman. He never had the success he desired and craved. He always had visions of grandeur and perceived himself to be something that he wasn't. He did not possess the charisma and skills necessary to be a truly successful salesman. He came across as contrived and fake. Because of the lack of skills, he was destined to failure. The fact that he did not realize this about himself only made his situation worse.