The mentioned mats from tomestones can be even more interesting when a new big patch with new gear is released. In the first week when there is high demand on those you can easily sell your capped tomes for maybe 2 millions instead of 200K*. Gemstone mats "could" also heavily increase - but i'm not 100% sure on this, i would rather cap them untill you see how the demand after the patch is, maybe they add new mats. Joining Hunttrains can also be interesting, with a good contribution you get about 12 stellaclusters for a full tour which you can trade for tier viii combat materia which "could" very likely heavily increase in price when 5.4 and with that a lot of gear is released.

When i mentioned "could" it's something that i expect but i'm not totally convinced of. Usually when a new patch comes there are certain cheap items that suddenly skyrocket on the MB. The trick is to get a clue on what will get high demand and taking some risk. ;-)

Another thing that is interesting is find some really good levequests for crafters. Sometimes you find leves that you can buy cheap on the marketboard and earn gil by delivering them (or crafting them by yourself). Also there is still some dungeonloot that gets sold for really good prices for example minions. Sure, it would be helpful to mention which dungeon and which minion but thats the issue with advices on making gil. The more know about the less they work ;-)

*Prices depend on dc's / worlds