Quoted from jalpert:
?. If someone started a thread to comment on an existing post because you don't feel the comment is appropriate, there would be a lot of posts
Because if you actually have a discussion worth having.. it will happen and it will take care of itself. If you feel the urge to just critique every for sale post with commentary.. yes you would have a lot of extra threads. But that should be a red flag to you to think 'is this really appropriate?'. Why do you need to share your commentary in the first place?
Do you follow everyone around in town you see and let them know what you think about the car they drive? Or how they dress?
Or do you make a point of telling the manager of every store you walk into what you think of their prices?
Do you do this dozens of times a day for everything you see... or if you see a trend.. do you have one conversation to discuss the TREND?
If you are concerned about prices for a game.. start a thread talk about the going asking and selling prices of a title. If there is something interesting to the discussion, it will carry itself. That is not the same thing as someone trying to correct someone's individual Ad because it doesn't fit how you think it should.
The first is a discussion about the hobby - the second is just people interjecting themselves as the peanut gallery.
Courtesy in online forums is not that much different than in person. You don't walk into every seller's store or stand and tell them what you think of their prices when you aren't even in the market for what they are selling.. so why do you do it here?