If you know that people usually needs only 5 of that item, why are you selling it in a 99/stack? And if you are able to buy loose pieces, do you think other people who's trying to get rid of their stacks of 99 is just gonna sit there while you undercut them?

Doing so kills of a dimension of the game where aside from pricing your goods, you can also decide to break bulk and sell it at a premium. So no.