You may have seen a great deal of guides to acquire enough FFXIV Gil for you to enjoy yourselves in game. Guides that introduced following are set out to turn you into one of the most skilled players on your server.
It's important to know the difference between transference of wealth, and generation of wealth. DoH generates wealth via Leves, and transfers wealth via Market Board. As for healthy practices, always use the transaction history to price your goods. Fast moving items should rarely, if ever, undercut, since you can price them on the same level as competitors without "losing" any sales.
Also, the transaction history will give you a great starting point for pricing your wares. For which trades are most valued, it's pretty much a toss up. Weaver has some of the most expensive materials, but their product isn't any more a commodity than say, Armorsmith or Leatherworker, which both have dirt cheap materials. Outside of that example, I can't think of any DoH that has a distinct advantage over another.
Also a level 50 goldsmith here. While you're right that the level 55 items hardly move and aren't worth much money, we still have some good change to be made in the level 48/49 accessories. A ton of people are getting up there, and a ton of people realize they need more VIT to take on titan, so you can sell the amber set for a fair bit. On my server, Siren, About 5 or 6 sets of Amber and about 1 of all the others are bought an hour during the weekend. If you're the one that has the lowest price on HQ (which is practically guaranteed as a level 50) you're looking at close to a 300k profit per hour on the weekends if you can keep up with demand(at least on my server prices for HQ goes from 6-8k depending on competition). It's a pain since you have to craft A LOT OF ITEMS, but there is defs cash to be made.