Everything that mshurn said in #2 is right on. The interesting part of this question to me is whether any of us has a choice about what happens to us. Of course there is some wiggle room in ths process, but so many things push us toward the decisions that we make, that it's hard to see how any of us totally "responsible" for everything that happens to us. Our education or lack thereof, family contacts, mental and physical skills, the ever changing world we live in (a big part of Willy's problem), and the quick decisions we make that seem harmless (the "buyer" in Boston), all these things "take away" our freedom, and with that our responsibility. It's hard for me to say this because I too feel that we are "free," but the older I get, the less freedom I see. It's not that were not responsible at all, just that we seem to be less responsible thn I once throught.
Theodore Dreiser and other naturalist/determinist writers explore this is some depth. "Sister Carrie," is driven like a "moth to the flame" in her search for fame and fortune; Hurstwood makes horrible decisions as he seeks to escape from his miserable marriage.
B. F. Skinner's "Beyond Freedom and Dignity" explores this issue in depth and would be an interesting read if you want to follow up on the behavorist approach to freedom/ responsibility.
I hate the idea that anyone lacks the power to overcome circumstances, at least to some extent, but considering Willy's family background, the time and place in which he lived, and his occupation, I don't believe he had a chance to live his life any differently.
Willy's values were framed by his family, his job, and his era. He valued financial success and defined himself only in those terms. Willy's father was a financial success, a rugged individualist and creative inventor. His brother Ben succeeded on a grand scale, and Ben's great success haunted Willy all his life. Ben "walked into the jungle" and came out a rich man. Willy stayed home to take care of his family and achieve success at home rather than far away, but no matter how hard he worked, Willy never struck it rich. He was not a risk taker. He had a family for whom he felt a strong sense of responsibility.
As a salesman, Willy lived and died by competition. He was never out from under that gun; he could never rest. He worked in an American era of an expanding economy. Everyone, it seems, except Willy, was making a lot of money, including his friend Charley. Unlike Charley, Willy had not changed with the times. He was the product of an earlier age who outlived his usefulness. Willy could not save himself. He could not change how he lived because he could not change who and what he was.
Willy is solely responsible for his own unhappy end in the drama. he is far from being an Oedipus. It is Willy's mistaken ideals, his stale cliches, his petty infidelity, and his deceptions, while suffering from a smallness of mind, Willy rises to a certain a degree of dignity through self-sacrifice in the end.