There's a problem with the "stability" of rampant undercutting:

NPC vendor prices do not change based on the market value of gil. They are static unless Square changes them. Currently right now the lack of a listing fee and the ability to freely change the price of anything without paying a fee has allowed the RMTs to intentionally crash markets, accelerating human stupidity to make everything worthless. When everything is worthless, players still need gil for things that NPCs sell--FC housing in 2.1 being a big thing that a lot of people will be wanting. It is very much in the RMTs interest to crash every market they possibly can. When every market is terrible, and nobody can make any gil from selling anything on the market boards, a lot of players will feel they have no option but to buy gil from the RMTs.