For starters you'll want all your crafts to 50 to help HQ things. In general, the following tends to sell well:

1: Quest/leve required items. As you already seem to have noticed, some folks are lazy and will just buy the item rather than craft it themselves. For leves you should probably craft the items used in the Charity leves, which are the ones that can be turned in 3 times and require 3 items per turn-in (5 for items crafted in batches of 3). Sell them in quantities of 9 (or 15). As for quests, don't forget about the relic quest. You may wish to sell the base weapon without the materia attached, since to make a profit you'll likely need to price it high enough that it will be at the bottom of the list.

2: Spiritbind items. This is where goldsmith shines, as binding only accessories and keeping the rest of your endgame gear on is a common strategy to keep kill speed up while spiritbinding. Zircon, tourmaline, rubellite, and amber accessories are the most used for battle I believe, although you can also make crafting accessories for people to SB as well.

3: High end meldable gear. It's common (or used to be at least) to craft high level accessories and meld vit to them so non-tanks can survive attacks. You need to craft the highest tier for this (90 atm), so it's only an option after pouring a lot of gil into melding a set.

4: Vanity items. Some items are made solely for glamour purposes, and furniture is more desired with personal chambers. This requires you to keep an eye on fashion trends however, as well as score some rather rare materials for a lot of it.