Dave Singleman was the man Willy admires. Note the symbolism of his name...Singleman...Willy longs to not be tied down by his responsibilities. Dave lived a long life, and was so succcessful that in later years he no longer had to go on the road (something Willy wants to do as well.) He had made a success of his career and reaps the rewards of money, contentment, and relaxation.
Here is the passage in which Willy explains his admiration:
His name was Dave Singleman. And he was eighty-four years old, and he'd drummed merchandise in thirty-one states. And old Dave, he'd go up to his room, y'understand, put on his green velet slippers -- I'll never forget -- and pick up his phone and call the buyers without ever leaving his room, at the age of eighty-four, he made his living. And when I saw that, I realized that selling was the greatest career a man could want."