While it's true that it'll eventually sell, this is because all of the other people selling said item have already sold theirs and now yours is all that's left on the market board. Again, yes, you do make the profit from it eventually, but it's largely based on how long it sits there and if other players decide to sell the same item. By this logic, you can put an item in your retainer for the maximum gil amount possible, and while it may take a long time, even years possibly, it will sell once no one else is selling that item....

Not really arguing with what you're saying, just pointing out a flaw with purposefully trying to make gil this way. More than likely in the time it took you to craft these items and sell them, you could have made just as much gil after costs by doing dungeons, FATES, and quests, while selling the excess items from those....