Dave Singleman epitomised success in advertising for Willy. He apparently lived the life that Willy hoped to emulate, and the description which Willy gives of him is where the title from the play is taken:
'when he died - and by the way he died the death of a salesman, in his green velvet slippers in the smoker of the New York, New Haven and Hartford, going in to Boston'
There is a sad contrast with Willy's death where the stage directions tell us of his dramatic breakdown-
He turns around as if to find his way; sounds, faces, voices seem to be swarming in upon him and he flicks at them, crying,
Just before he drives to his death.
Also, we are told that Dave Singleman's funeral was huge,
'hundreds of salesman and buyers were at his funeral'
whereas Linda asks why there were so few people at Willy's.
We can imagine that the character of 'Singleman' is unique and venerated in his profession, whereas 'Loman' is invisible, inferior and easily forgotten.