The best 'make money quick' methods I've ever seen were to take advantage of patch/expansion cycles. When housing wards opened up. I had 8 retainers all selling furniture. I made over 200million gil the first 2 weeks of 4.1 and 4.2 (whatever the two housing patches were). The same tactics can work on other areas too. New crafting/gathering set coming out? You bet your balls that crafting/gathering materia will triple in price! I probably bought 100m~ worth of crafting materia grade 5s at 20-25k each. Then when the last patch came out. Most of them sold for 150k+!!!

I quit to play Warcraft, but I'm still passionate about the FFXIV market (something WoW doesn't have). If I was more invested in FFXIV, I would recommend looking at material prices. Maps have caused some items to plumit. Consider buying cheap and selling high during a patch cycle. Find out what materials are valuable--invest 100-200m into them when they're cheap. Then resell for double at a later date. It helps to make money if you actually have money to invest. I tend to enforce a min gil amount. Say like "never go below 50m," because you never know when someone will flood the market with super cheap materials--and whoever buys it all up--wins a free pay day later.