The easiest way to make gil without having to do much of anything is to send your retainers out to get you things that sell well.
In the case of combat retainers, leather and certain alchemical reagents are likely decent sellers.
In the case of gathering retainers (which requires you to level gathering jobs and to gather each item you want your retainer to get at least once), you can send them out for ores/plants/fish currently in demand.
For any retainer there's also quick ventures which are pretty hit or miss but can come back with some valuable things to sell. Certain housing or glam items they can return with will sell quite well on the MB. There's also the 18 hour exploration ventures. While not the most efficient means of earning gil, they do come back with allagan coins you can simply vendor for gil.
There's also the Doman Enclave weekly turn ins. That can net you 40,000 gil a week. I like turning in 4 clear demimateria for that since I have a ton of them and nothing to use them on.
Even if you aren't a fan of collectibles but can stomach doing the weekly Deliveries (which doesn't take very long or require high end gear), you can use the scrips rewarded from that each week to buy some crafting or gathering materia to sell on the MB
If you're drowning in GC seals like I usually am, and have hundreds of venture coins and cordials already, you can also buy housing items to sell on the MB with them or even the tradeable items for the old HW relic which still sell for people going back to collect them.
If you play a job that can take advantage of the adventurer in need bonus for roulettes that award crystals used to buy materia, you can sell that materia for decent gil, especially the grade 8's.
