Some legit reasons to undercut:

- Bulk discount. If someone is selling a single for a high price, I will sell a stack of 99 for a chunk lower. Maybe a person only needs 1 of the thing, but if they need 99, I'm gonna be the go-to.
- Everything listed is HQ and I have a NQ of the thing. It's gonna be half the price of the HQ in that case, regardless of sales history.
- I got that thing from a retainer quick venture or a quest, and the effort I put forth into getting it was 0, so I don't mind losing potential money to sell it faster.
- I made it and I know how little effort it was for me to make and gather the materials, and the listed price is WAY too high for the amount of work it actually took a competent crafter.
- There are 20 items all listed by the same retainer in their attempt to flood the market. I'm gonna undercut by at least a few gil to get listed at the top.