Arthur Miller wanted to write about the common man and how he is exploited and alienated under capitalism. But a stage play relies almost exclusively on dialogue, and the typical working man is not talkative, especially about his feelings. Miller must have chosen to write about a salesman because salesmen are working-class types but generally outgoing and talkative. In fact, Willy Loman talks so much that nobody in his family can get a word in edgewise. He has been talking all his adult life but seldom listening. David Mamet's excellent play Glengarry Glen Ross is also about salesmen. Mamet must have been influenced by Arthur Miller's play. 

The salesman purports to sell his product, but is in fact selling himself. We never know what product or service Willy is supposed to promote, only that he is unsuccessful at it. The implication is that the product is irrelevant: it is the image and the lifestyle which accompanies it which ensures success.

The salesman figure is an example of a person who does not need professional training or education to do his job. He can come from anywhere, any background, and this makes him a prime figure to exemplify the "everyman". 

Also, there is no ceiling or limit to the amount of money a salesman can make (speaking theoretically). A person can sell one car or one insurance policy every month or a person can sell ten or a hundred or a thousand.

This possibility and self-determination feeds into the idea that the salesman is a natural dreamer, succeeding or failing according to the limits of his/her own abilities and motivations but driven to dream of "what could be" regardless of success or failure. 

If I understand you correctly, you are asking about the salesman as a symbol.  I think that salesman easily stand for greed and barely attainable goals.  This is somewhat stereotypical, but a salesman’s goal is to sell at all costs.  Sales is cutthroat no matter the industry.  It all strikes me as somewhat hopeless, so I think the salesman was an appropriate symbol for a play on hopeless ambition and crushed dreams.