Other people gave good advice for the low hanging fruit. I am going to explain the MB a bit more to give a picture of why this sorta advice is hard.

A big issue with any method of making money via the MB is that the MB is the most 'competitive' element of the game. Not everyone can be a winner there. While its true any given item may have an extremely good profit margin there, that isn't the only factor to think of in MB sales. If it was, I could say "Oh yeah nab a bunch of X and just sell it." A second critical element of the MB is how much demand there is, meaning not only am I discouraged from sharing info, my info can get out of date very quickly as everyone who wants something nabs it.

This is doubly hard for a gatherer, as the barrier of entry for a gatherer is way lower than a crafter, so markets crash fast. You can certainly make money on key stuff that people are using, but it won't be very much very fast. Worse, almost every crafter is also a gatherer, and while they are willing to buy items they need if its faster and will save them money because time=money, often times the profit-time ratio for the gatherer on truly 'staple' gathers is awful.

So you need to understand the MB at least a bit. Here are a few good practices to making cash and staying flexible.

1: Try to understand how an item is used. If an item literally only ever is used in a given number, sell in that number unless the item drops in a way where someone might want to just 'complete the set.' Understand if the item in question will have ongoing demand from even endgame crafters or if its a springboard. Take notes on what sells and when, check the past sales and make sure your not getting taken for a ride.

2: Look for things players might struggle to get themselves, either because its hard (ex: HQ stuff for job quests), or because its just a pain (Ex: A hidden item in a node). These items bypass the 'flood' effect of how common gatherers are and are good Gil to time simply because getting a stack of these is a pain in the butt. So they can be very good gil to time

3: Don't overflood a given market. If you have 99 of something that sells in batches of 4, don't drop 99 of them, or put each one up on your retainer all at once. That is a good way to dramatically flood the market and crash it, ESPECIALLY with lower level items.

4: Don't dramatically undercut unless you want to crash the market. Try to restrain yourself and just undercut by 1 gil, unless you want to dump products rather than bother with them anymore (again, time is money, so micromanaging something that isn't worth your time is costing you cash, so it isn't lazy to do this). Then undercut by a *lot* so people will just buy what you have and add it to their stock.

5: Related to 4: If someone undercuts a stable market by a lot, and you know that item sells well, don't match their pice, just but it and add it to your stock.