Level up gatherers and sell the mats. They sell often but maybe not for the highest cost. Especially if you seek out Levequest items and GC turn in items and gather them in HQ. Consistent income

Retainer Quick Ventures can sometimes be fruitful in what they return.

Instead of vendoring, go to Doma and donate for a multiplier bonus to your vendor amount, resets each week.

If playing through the msq you get a lot of HQQ gear you can sell off. Just check the Market board for the best item to sell off of the available options for the quest completion and pick that when you finish the quest.

Likewise, in Shadowbringers you get a coffer for a specific level and gear piece. It is based on the job you are when you open it. SO if you get the level 78 head coffer and the casting head piece sells for the highest, open it as a caster and sell. If it's dragoon, open as a dragoon and sell. Likewise for all the other coffers you get. Takes time but can net you a lot of income and is also a good free GC turn in/Leve quest turn in if you don't need the gear. It can take time but if you have some time spare and some coffers left just browse the jobs and see what sells highest. Open in that job and sell it.

Spend hours of your day browsing the Marketboard looking for that 1 person to miss a 0 off the end of their listing xD Night Pegasus selling for £3m? Don't mind me lol - semi serious suggestion.