So, having leveled some crafters thanks to the magic of the Ishgardian Restoration, it's time to figure out how to make money! Now, I am a newbie in this field, but I feel confident in saying:Do not sell finished goods!
Unless you are an experienced crafter, armed with knowledge, willingness, and heavily-overmelded gear, I think it is best to avoid selling finished goods for profit, and instead sell intermediate crafting materials. These are crafted items whose main use is as ingredients in other crafts, such as lumber, nuggets/ingots, circles of leather, cut jewels, etc. I think selling intermediate materials has many advantages:
1) Intermediate materials are easier to craft, and do not demand high stats. In fact, some ARR/HW/SB materials still sell well, and can be easily crafted even without any endgame gear. So, you won't have to invest in melding a lot of expensive crafter materia. Conversely, current battle/DoH/DoL gear, raid food and potions all require very high stats to craft and HQ successfully, and only the HQ versions are valuable.
2) If you sell finished goods, such as a housing decoration, you can only sell it if someone wants the exact item you are selling. On the other hand, if you sell intermediate materials, sometimes the same item can be used in over 100 different crafts, so it is more likely that people want to buy it.
3) Selling crafting materials in small stacks allows you to compete even against others who can produce the item for cheaper. Let's say you want to sell some lumber, and the cheapest listing on the market is a stack of 99 for 1,000 gil each. You can sell a stack of 5 at 3,000 gil each and still get sales, because there are people who only want a few of the item. On the other hand, if you sell a piece of battle gear, the only thing you can compete on is price.
4) The penalty for making mistakes when crafting intermediate materials is much lighter. If you are not confident working with 200k worth of materials where 1 wrong button makes your money go up in smoke, it's best to stick to low-risk items.
...Of course, if you do want to craft finished goods, one thing you can do is to craft collectibles and exchange them for white scrips, then buy white scrip items to sell (Sublime Solution sells well as it is used in a ton of crafts, though there may be heavy competition on price).
In order to maximize profit margin, ideally you will also have leveled gatherers to gather the items you need - MIN for BSM, ARM and GSM, and BTN for CRP and CUL. ALC is weird, and their alkahests, glues and solutions can require both miner and botanist items, as well as monster items. If you are LTW, make sure to have lots of combat retainers and ventures to supply yourself, because going out and actually killing monsters for skins is not efficient. It's best not to rely on the market board for supply of raw materials. However, it's worth checking it from time, since occasionally people dump huge amounts of items they don't want anymore at a low price.
When choosing things to sell, I would suggest looking at things with ingredients that are annoying to gather - items that require materials from legendary nodes, hidden items, materials purchased with poetics and GC seals, etc.