Quoted from Whysnow:totally fair game and EXPECT comments if you make a FS ad and forum topic with it.
The thing that matters is how you say it or your motives behind it. For example, certain people are well known for crapping on a thread to try and push people off a game or down play its value because they are actively hunting for the title.
I just prefer some linking of actual sales (like forceflow did in the Jolly thread once called out for lack of it). It is especially important when dealing with a rare deck and few sales. Just saying "do your research" is passive aggressive BS which can call a seller or game into question without merit.
TLDR... intent of your comment is just as important and using facts/past sales makes your opinion have some value.
Lets be honest here you are talking about me. I would also say you have a tendency to price pump machines you already own, but then if we get into this debate I will get another long winded rant of a pm from you that will just get ignored.
You don't back up your opinions on sales unless called out and even then from experience you have used only ones that support your view. So before you start pointing fingers at me why don't you live up to your own expectations.