I understand these writers' annoyance at this question. To them it feels like someone is asking a more personal question than one should.
I STRONGLY urge you to look at this from a different perspective, particularly if you are self-publishing and thus your very own sales department.
There is only ONE correct answer to that question:
"Just great, thank you for asking!"
Because people are your buyers. Even people who ask these kinds of questions. And people respond to positive encouragement. If your book is doing well, clearly the implication is the person should buy and read it.
But more than that, it gets you in the habit of being positive about sales.
As the face of your book in the world, you want to get right to the heart of the matter too: everyone should buy and read your book, so of course it's selling nicely, and here's a bookmark with how to order.
Someone asking about your book is an opportunity, not an intrusive personal question.
Talking about your book is business NOT personal.