This is a "test" question to see how you would approach the objective, which is to sell the interviewer the pen.
First of all -- describe the FEATURES of the pen. Its portable, fits the hand, has the right color ink, retracts so that it does not splotch your pocket with ink leak, etc.
Second, relate each feature to a user benefit. The right color ink means you don't get caught with red when you need black; the portability means you can carry it everywhere and anywhere. The shape fits the hand so you don't get carpel tunnel syndrome from holding it -- have you tried to hold one of those huge pens we sometimes see around?
Now, ask if the benefits of the pen means that he will be able to do his job better, be more effective, more successful.
In short, you are working to relate each FEATURE of the pen into a DEFINED BENEFIT for the user. The user needs to recognize that owning the pen will make him more successful, more fulfilled, more efficient, more ---whatever you can think of.
Hope this helps.
As a sales manager, I used to do this all the time. It is a good exercise for seeing if someone has natural selling skills, and is not afraid of talking.