Playing a tank is also helpful for generating cash, since they usually add the "adventurer in need" bonus to use the dungeon roulette. Low level roulette tanking not only adds a ton of exp but also nets you a ton of cash (7500 gil bonus at level 50). Even if you don't want to main a tank, playing one just to have the adventurer in need bonus is lucrative.

I would also add to remember to do your Guildhests for the first time for each class - guildhests are quick and the first-time completion bonus gives a lot of gil and you can get that bonus for each class.

And lastly, you mentioned the challenge log but it should be emphasized that it can get you 20-30k gil each week pretty easily. It forces new players to do different activities (dungeons, fates, guildhests, leve-quests etc.) to get them used to all the different content and also boosts their exp a ton so not only are they getting a lot of gil but they are leveling quicker too.

The good thing about these suggestions is that they all involve raw-gil generation and don't rely on selling stuff on the market board.