What is your ACTUAL reasoning for undercutting?

[Windsun33]

And then, one day when I checked the market board again, I see in the history that someone else had put his bikini skirt on the market board and had already sold it for 10 gil less than mine. It was only a few hours since I checked the last time.

Did he actually SELL it at that price, or was that just the price on the MB. If you look through some of the mid to high priced stuff, you will find many items that simply never sell - possibly for some because nobody knows they exist (like maybe your red bikini?)