"Why are you selling?" is a genuine question I think anyone would ask when buying a car if not already stated. Whenever I have sold cars I would always note the reason for sale, whether it be "I just fancy a change" or "I require something more practical" etc.
I think codel pretty much nailed it anyway. I hadn't seen the thread in question until just now and I do get how some of it is kind of unhelpful. You're going to get it anywhere though and to be fair, the comments on this forum are nowhere near the texts / emails you get from the eBay / PH moron brigade. The forum provides all the mechanisms to avoid the above if you so wish, I can't really see where the issue is....
The why are you selling didnt get asked, he got asked, why have you got two cars, are you a trader etc, perhaps fair comment if you wanted to BUY, but it was already said, one was his, one was his girlfriends.
Or just someone asking the question, getting an answer, which then highlights the reason for selling to a potential buyer? It's similar to (although less of an issue) than someone forgetting to write how long until the next MOT for example... Nobody would mind if the question was asked then by a non-buyer.