1) True, but it starts a baseline and then one can move it down as Ray said. It establishes the local market. Also, if someone lists X for $Y and it goes in 20 minutes, one might realize its local value is higher than they thought (though the outlying factor could be the buyer is stupid, really wanted that item, etc.).
2) Do you think they don't already know this? Unless one is doing (illegal) FTF sales that data is available if they want it.