I find it funny that the OP makes this sound like your reasoning for undercutting a price must include some kind of personal feelings towards the other sellers. If I decide I need to sell something for 500 gil less than the current offering, I just care that the numbers and the sell rate of the item tell me that I will likely be able to actually sell my item in a timely fashion at the price I'm setting. It has nothing to do with the other sellers. I don't even know or care who they are. Do other people actually bother with that information???