What is your ACTUAL reasoning for undercutting?

[Orbus]

Time is money.
People who think making an extra thousand gil if they leave their slightly overpriced item on the MB for a month is good strategy will never make the big cash.
Most people only have the two retainers, forty market slots is a real bottleneck if you're a crafter/gatherer.

Here's my free tip, because it also benefits me when people stop doing this.
Nobody wants to buy your stack of 99 muguffins.
Unless it's a very current game material and even then your market will be limited.
Stacks of five or ten (or one!) will ALWAYS sell faster even if your upcharging out the arse, because that's still hella cheaper than 99 of them, which is way more than most people want or need anyway.