What if they put averages, based on weekly sells, so people could not put anything under a certain amount...
Now I realize that this limitation will upset a lot of people that want to sell things as fast as possible, but it would also give people a chance to sell things for a decent profit. And in turn have some gil circulating.
You would not be able to NPC the said item for the Minimum amount of gil.
But setting minimums on certain items based off of averages that reset weekly well, and at the beginning of a month it would reset completely. People might rush to sell things at the beginning of the month and that might be spike sell times.
Either way this is a very rough, idea, that would need to be refined, molded and worked over by someone that has a much better understanding about economics...but at least it could be a start.