I always laugh to myself when I see posts like this. Sorry. But here's a quick tip:

If people post the answer in an open forums like this, then EVERYONE will be doing it to make money. Suddenly, the market is flooded with specific items that won't sell, now NO ONE is making money.

Case in point, when Mithrie posted how he made millions of gil with Mastercraft Demimateria, the market was suddenly flooded and the Mastercraft Demimateria prices plummeted.

My point is, you have to find out for yourself how to do it.