It is an interesting (and ironic) reality that, in a play called Death of a Salesman which has a salesman as its protagonist, this question must be asked. Arthur Miller could certainly have made it perfectly clear to us what Willy Loman was selling, but he chose not to do that. The obvious conclusion is that if the reader does not know, it probably does not matter.
We do know that Loman carries "two large sample cases" with him on the road, that what he sells is part of a line of products (he mentions he has a meeting "to show the line" to a company), the products are sold by the gross (he mentions "two hundred gross" in one order), and he works for the Wagner Company. That is not much to go on if one were to make an educated guess about what Loman sells.
The fact that we do not know what Loman spends his life selling is fitting, because Loman does not believe that what he has spent his life doing matters anymore. It does not. He has always found his worth in his accomplishments (success as a salesman), so when he is no longer successful at work (not to mention the mess he has made of his family relationships) he can find no reason to live.
What Willy Loman sells does not matter because what Willy Loman does does not matter--at least not enough to inspire him to keep on living.