I think it depends. Some people deserve it. Yes it's a jerk move to say something like "The average price of the last three on Ebay is $23.50" when the guy wants $26 or something.
On the other hand, take this new thread for example:
This guy it letting us all know about his amazing high-end quick sales with his firm, rock bottom prices that are 4x times what the cards sell for.
And he is totally above ridicule? I don't think so. In a way that belittles everyone on here as if we are not going to notice the price differences because he is hooking us up with great quick sale deals.
The above are two extremes however. If it's somewhere in between I think a helpful suggestion can be okay, maybe not be super mean about it.
Supercollecting Josh Johnson (University of San Diego, Tampa Bay Buccaneers, Cincinnati Bengals, San Francisco 49ers)
Collecting Philip Rivers, Padres, Packers, Lakers, University of San Diego alumni
My Bucket, always selling and trading: http://s1204.photobucket.com/user/kramerattack/library/