This is what I do, except by several thousand gil, whenever my retainer brings home some valuable piece of furniture or materia or whatever. I'm not trying to run a business, I'm just trying to clear out my inventory quickly. Leaving a few thousand gil on the table doesn't bother me since I already have more money than I know what to do with. If someone wants to purchase my item and then relist it at a higher price, more power to them.
It mostly just amuses me that someone would get mad at me for listing a materia for 30,000 gil, because that infringes on their right to sell that same materia for the "correct" price of 40,000 gil or whatever.