When it comes to earning gil in Final Fantasy XIV, the market board is always going to be one of your greatest tools. As a completely player-run marketplace, it's anyone's game. To make gil selling your items, all you have to do is make them worth it to an adventurer in need.
As you might expect, you can get pretty underhanded with this. Ultimately, flipping items on the market board is a game of buying low and selling high. It's a way of turning a profit with minimal effort. There will always be players who will do anything to get the most out of their gil, and players who'd rather take the convenient option. With this guide, you'll be a little closer to being the former.
For all of the following tips, players can use Universalis to check market board prices. Otherwise, you'd have to go to every World on every Data Center available to you to check the prices for every item you're considering selling, which is time consuming. Checking them all in one place is the better move here.
How to make a profit by reselling vendor items

Some of the easiest, safest items to resell on the market board are items sold by NPCs. It can be a pain for players to figure out just which items are available to purchase directly from vendors in FFXIV, so many of them opt to just run to the nearest market board to grab what they need. This is perfect for beginning players in need of gil, as many vendors are available early in the game.
The Housing Merchants in every Residential District have some really great options, as furnishings are always in demand. However, there are countless other NPCs selling items throughout the game. It's always a good idea to check on what items they offer and see if any of them might be good for flipping. Beyond that, items that can be bought with other currencies (White and Purple Crafters' Scrips, Sacks of Nuts, Company Seals, etc.) can generally be sold for a decent profit. After all, farming for the various currencies can be a pain compared to grabbing something quickly off of the market board.
How to flip market board items on your own World

Of course, you can always take things from the market board and put them right back up for sale. This is generally going to get you bigger profits than selling vendor items, but it might also require you to put more gil in to start, and requires basic knowledge of economics. Even without leaving your home World, there are usually some items up for sale that have a sizable gap between their lowest available prices and the mid-priced options. Say there are two of an item available for 19,000 and 20,000 gil respectively, and the next-cheapest option is 40,000. You could grab the first two, and immediately list them both for 39,999 each. If they both sold (and they'd be likely to, as the cheapest options available on that World), you'd more than double the gil you'd spent.
It's a pretty low-effort option, especially if you find yourself checking on the market board on your home World often anyway. Depending on when you happen to look, you might just find a way bigger gap to exploit. Just make sure you look at the purchase history and make sure the item you're looking to flip sells consistently, or you might find your items sitting around collecting dust.
How to flip market board items across Worlds

Thanks to World and Data Center travel, you can take your market board flipping to the next level if you're willing to put in a little extra effort. Universalis makes it easy to compare prices on items regardless of World, expanding the hunt for the lowest price to even distant Worlds and Data Centers. There's nothing stopping players from traveling for the best prices, but many won't do it for the sake of convenience. This leaves the best bargains ready for the taking.
Prices will almost always vary the most between Data Centers, so if you're looking for the best chance at turning a profit, you'll want to cast your net as wide as possible. What you're looking for is an item that's selling high on your home World, but low on another World you can grab it from. Head over, buy the item for cheap, go home, sell it for the higher price, and repeat as necessary.
Things to keep in mind when flipping items on the market board
One thing you'll want to keep in mind is that tax rates will vary. Between Worlds, between city-states, and even over time, these rates will change, and it's in your best interest to check what they are before you try to make any big sales. The lower the tax rates, less gil you'll lose to taxes per sale, so try to dispatch your retainers to a city-state with reduced rates if you can. To check the rates and switch your retainers' city-state, just talk to the local Retainer Vocate.