The assumption is that the new hire better be able to do it or they won't be hired because it "obviously" shows if they are a good salesperson.
But to me this question misses whether they are a good salesperson or not. Building rapport, finding something in common and hearing why the customer needs or wants this pen are all assumed - yet those parts of the process are what make or break a sale.
It also assumes you can "sell" anyone something. I believe it was Zig Ziglar who said it thirty years ago, "Customers hate to be sold but they love to buy."
Analyze what questions you are using to interview potential employees and what both the question and their answers are really showing you.
Do you know what qualities you are trying to discover with each question?
Do you know what the "A," "C," and fail answers would look like?
Do you have a list of ten questions that you ask everyone so you can fairly judge their abilities?
If so then you are more likely to make a good hiring decision. But that's hardly the goal.
Because the chances of making a good hire are pretty much the same as a coin toss. Until you get them on your sales floor working in your system with your people, you simply don't know how they will respond.
So is the employee interviewing question "How would you sell me this pen?" such a litmus test of someone's ability on your floor? My guess is not so don't cut them off if they can't manufacture some "assumptive," "porcupine" or alternate of choice" closing technique on you.
As a Driver personality, if you did that to me, I'd be insulted and walk out. And Drivers are the most likely to be able to make the sale for you.