Not true at all. In your "looking out for oneself" scenario, someone has a shot at a game for a great price. In your "looking out for others" scenario, nobody gets a game at all.
In the process of looking out for one person, you are actually screwing another.
Again it seems that you depend on extreme examples, and that someone has to be blamed. Others have posted some good arguments that should be considered for not posting.
For example, last week a friend recently picked up an AFM for 5600 because another pinhead in the area let him know directly that he found it on CL. Had it been posted, surely I would have tried to jump on it as well as many others. In this case, it is a real world example of how it can be beneficial not to post. I am glad he got the game, and at the same time, I wish I would have had a shot at it.
Being new to the board, I have posted a CL ad because several posted that they were looking for the game in the ad. I wasn't aware of the sensitivities that some have on this. Now that I am better informed, I probably would follow the example of my friends above.
But I wouldn't be dogmatic about it.