Got my first million in in game (from the 30 day reward thing but my first million of my own work was) from buying steel sallets, toadskin armguards, linen tights, linen bliauds, mythril sallets etc all from NPC for 1-3k Gil and selling on mb for 30-40k occasionally for some reason linen gaskins of gahering would sell for 100k (not sure why but people were paying it) over time I made my money.

Not sure it would work now though as at the time louisoix was new and had lots of new players and so all that low item stuff was in demand. Now some of those gems are selling for only a few hundred gil so not as profitable.

Retainer quick ventures can occasionally bring you back a gem between the mountain of fish they collect too.