Earning large amount of Gil

[Tint]

and keep in mind that the market board is kind of a backwards auction. people will likely buy the cheapest item, wich is always on top of the list (even when they have to pay 1000 gil for the taxes only to pay 10 gil less for the product - no, it doesn't make sense), so you have to check prices often (once ever 1-2 hours is fine) and adjust. if you want to sell items fast you have to undercut often for a few gil, just to stay on top of the list. the faster you sell an item the more items you can put on the market, since you only have 20 slots per retainer, space matters.
an item sitting there for a week for 10k gil is worth nothing. you could have used that slot to actually sell something. higher priced items obviously can stay longer on your retainer without losing potential money.

also check what the items you gather are actually used for: http://na.finalfantasyxiv.com/lodestone/playguide/db/
it's worth to put stacks in quantities on the market wich can be used for exactly one craft. if a crafter needs 3 ore for one craft, then put 3 ore in one stack on the market. or 6, 9, 12... you get the idea.
you also can put 4 or 5 into one stack, when there are no smaller quantities. the crafter will be forced to buy more than he needs, so you will get more money. but don't overdo it, nobody wants to buy a stack of 20 items when he only needs 3...

small stacks sell fast, and usually for more money / item. but you have to replace the sold items often. some people like to buy bigger stacks, so you can put a few on the market too.