Willy Loman is an unstable man. His job, his health, his mind, all fit perfectly into the category of "unstable." When thinking of the average working man building a secure home for his family, Willy Loman is not what comes to mind. Reasons for this include the fact that he attached a rubber tube to the gas pipe in his house in order to inhale gas... and his other efforts to try and kill himself by crashing his car. When someone is trying to secure and keep his family safe, these are not actions typically taken! Willy Loman is selfish. Even in his efforts to make his children into something great, he does it for himself, for his appearances. He wants to prove to outsiders looking in that he has made SOMETHING, ANYTHING of himself, even if it happens to be through his sons, Biff and Happy. They are tools he tries to use for his own ends.
The average man has limits. Mr. Loman seems to lack those limits, and it turns out to be detrimental for his family. Also, the average working man knows his place in ways Willy Loman seems to not know his. He is rude to his superiors, complimenting them in one second, and insulting them the next! Willy's bipolarity is something his dear wife Linda should have looked into, although it can be seen that she had quite enough on her plate.
Yet, Everyman has big dreams like Willy Loman. Everyman endeavors to bring home as much money as they can for their family, and Everyman has vices. Not Everyman cheats on their wife, but Everyman is influenced by temptation. Everyman wants the best for their children, and Everyman is self-conscious. Everyman is not perfect, and Willy Loman isn't either. In these ways, Willy Loman represents Everyman. He falls victim to societal expectations that only the very few can ever accomplish and yet just like Everyman, Willy still tries. Indeed, the American Dream is rarely ever achieved but Everyman tries.
Willy Loman is more than Everyman though. He represents the failures of the American society. He represents a betrayal of modern-day values (although admittedly the values were probably synonymous with the times in which the play was written), and he represents the trials and tribulations low class citizens were known to experience. He held the naivety of previous generations while being introduced to a new age, and so he faded away because of a failure to adapt. "You can't teach an old dog new tricks." Willy Loman was an old dog.