I don't exactly undercut per say, but I do match the lowest even number price the item is selling at. I do this because plain and simple. If I have something I don't need, I just want it gone from my inventory. If an item is selling average at mid tier at 1500 gil and the lowest price is around 1000 gil. I'm not losing sleep over a 500 gil difference.

My buying habits are pretty much the same. If an item I want at the lowest price is selling for 1000 gil followed by 999 gil and 995 gil. I'll always go for the 1000 gil selling price. For much the same reason, am not losing sleep over paying a whole whopping 5 gil more for an item.

My reasons basically come down to gil is pretty much useless to me in terms of what I need to play the game. I buy the occasional outfit, minion or housing item yes, but it's not something that's really needed to play the game. There's nothing game play wise that I need to maximize gil making for or cut my spending to save as much as I can.