The market is player driven. Players dictate value. The other day, I dumped 50 craftsman competence III on the broker for about 25% less per than the cheapest listed. I was sick of selling them one at a time and playing the "undercut by 1 gil" game. Someone bought them up and re-listed them in 4s and 5s for about 5% less than the market rate. Point is, that player saw opportunity in my undercutting and selling in bulk.

An item is only worth what someone will pay for it. Just because everyone else is trying to sell an item for 1000 gil doesn't make it worth 1000 gil. Maybe today players are only willing to pay 80 gil. Maybe tomorrow, 5000 gil. You can't enforce what the OP is talking about.