This isn`t the thread for a debate on "Collecting/Hobby vs Investment", so I`ll keep my comment as short and close to the point as possible.
There are folks who fit solely into each of those categories, and others who sort of span the gap and fill both. If your goal is to solely make money, and who do not give a second thought to collecting or building, your perspective is going to be different than someone who fills the opposite category, and vice-versa. I`d also argue there are several sub-categories under each of those. For collecting/hobby: completists, minifigures collectors, entire set builders/collectors, sealed box collectors, etc. For investment: mainly flippers and long/longer term holders. I`ll focus on the second set of categories for the moments. Some people view this as a means to make quite a bit of money through the long term, thus holding their sets long for maximum profit or in other words increase wealth/capital. Others do this on the side as a means to make a quick buck here and there. Even still, there are plenty of people who aim to sell as a business and in most cases flip/sell other non-Lego items to do so. It should be obvious that everyone is going to approach this, like almost any other investment, through a different POV, sometimes a drastically different one. To say that someone else`s decision to buy or sell is stupid/insane/wrong only works if you understand the approach they are taking. As an example, someone buys a set for $100, sells it 6months later for $150, knowing full well it could be worth $200-$250 a year later. If the goal of that person was to maximize profit while doing minimal work, then it would be justified to say their choice was stupid. Of course it was! Why would they do such a thing? Unless perhaps they had different goals. They wanted a short term flip for optimal cash flow perhaps, and in that 6month span were able to flip more product with that cash and thus exceed the $50-$100 difference. There is just no way to tell for sure.
Bottom line there are different approach, and you have to look and see where someone else is coming from.
Now of course, there is just plain stupid, and there is tons of that around, far more than the presence of reason I`ve mentioned, but hopefully someone gets the point. Don`t always assume someone doesn`t know what they are doing, 8/10 times maybe they don`t, but 2/10 just might, and it`s not fair to toss them in with the rest.
And also, it really is tasteless IMHO to insult someone out of ignorance because you (generally) don`t know where they are coming from. It`s fine to not know, but anyone who wants to point a finger at the other guy should maybe stop and consider that they just might hold a perspective that is valuable/optimal for them, even if it is not in agreement with your own.