Something else to look into are Levequests. There are usually one or two per tier that have very easy material requirements and will net you a decent amount of gil; especially if you can find easy ones with triple turn ins.

If you don't mind having to go back out and collect materials to make gear as you level up, sell everything you get as long as it's going for at least 2x price you can get from a vendor. This will make sure you're not actually losing money due to market board fees. It may not seem like much at the time, but every little bit helps.

Unless you're using the materials for GC or Levequest turn-ins, sell all the HQ items you gather as they sell faster and make more money per sale. You'll be out leveling your personally crafted gear fast enough that making HQ versions for personal use isn't worth it. If you can reliably HQ these items, make a couple extra to put on the MB, chances are someone will buy it.

ALWAYS hand craft items. This will give you a better chance at making HQ items, will net you a lot more EXP per item which will get you leveled faster, which will allow you to make more lucrative items more quickly, etc, etc, etc.

Don't get caught up playing the Undercutting game. Find what you think to be a reasonable price for an item that isn't the lowest on the list, but isn't the highest either, and let it stay there. Demand is usually lowest during the week so people drop prices in hopes for sales. When demand goes up on the weekend chances are good that it'll sell. In some cases it's worth adjusting the price every day just to sell low-value NQ stuff to free up space to sell other things, but normally doing an adjustment on Sunday morning and sometimes mid-week helps. If you think something's valuable enough from your POV, it's worth leaving on the MB at the price you want to sell it at then wait for it to sell. It can take a while sometimes, but it'll eventually sell. I wouldn't use more than 5 slots per retainer on items like this though. Keep that low value stuff moving.

Make use of your retainers. Do as much content that gives you GC seals as possible which you will then use to buy Ventures and keep your retainers out in the field gathering things. Most of the time Quick Exploration doesn't results in big-ticket items, but sometimes it does, and with a 1 hr cool down, you can get a number of them done each day. Doing Hunting ventures is a fantastic way to target items that either sell well or can be used to make easy to craft items that sell for a decent amount.