Ya, I hate this, and don't understand it, but I have a few friends who all do this.

Something sells for 50,000g, and they will under cut it to 1,000 or less.

I'm like "Why?"

Them "I just wanted to clean my inventory out, I dont care about the gil."

In almost every scenario, its always because their bags are full, and u can only sell so much at a time.

So technically, the real enemy is inventory space, and limited AH sales.