I was joking a bit about how convoluted that route is, but seriously, marketboard sales ultimately funded by seals has been a steady source of gil. You don’t need to be doing big trades if you load up your retainers regularly (I load them up every week, more or less) with conveniently sized stacks of moderately priced crafting materials. They sell best and most consistently in my experience. Leveling roulette pays out a fat stack of seals for whatever role is needed. I have more than 300 ventures saved up and keep my retainers running while I run duties.