For items that you can gather directly, you have the choice of either going out to gather them yourself or to buy them from the AH. The former adds no cost to your item but requires time and effort, while the latter can be quick and hassle-free. Even should you choose to get the materials yourself, don't forget to add them to your cost — your labor and time should be compensated for, even if just a little.
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Skinning is the third gathering profession and it is very different from the other two. It works by skinning the dead bodies of animals with skins. You need to both kill the mob and then gather the body. Your main customers will be leatherworkers. Skinning in Legion is very good and gives a large amount of Blood of Sargeras. I suggest pairing this with a crafting profession that can unload a lot of bloods, like Alchemy or enchanting. For an overview of the best farming spots in Legion you can check out this guide by hikons or this one by WTBGold guides.
If you bought Legion and have no desire to go through old content, good news! The new expansion comes with a free boost to level 100 for one character. You can buy these boosts from the in-game store whenever you want, but they normally cost $60, so savor this free one. Use it for a class that you are confident you will enjoy. As a fresh level 100 character, you will be able to immediately begin the quest that takes you the Broken Isles, the new region where Legion takes place.
Blacksmithing is a crafting profession that uses bars obtained from melting ore to craft plate armor and weapons, as well as some assorted cosmetic items. In Legion you craft ilvl 850 gear (demonsteel), a mount and a legendary. These are the main gold making opportunities with blacksmithing. Blacksmithing can also create a lot of transmog gear from old expansion. Some rare recipes have looks that are very sought after primarily for weapons.
This is a risky way to make money, and not recommended. Tell a player that if he beats you in a duel you will give him 10 gold, but if you beat him he has to give you 5 gold. This can be very effective if you are skilled in PvP and make the bet with a trustworthy person, or friends. It is a very risky strategy, as you may lose gold, or the other player won't cough up the money anyway. You will also get a bad reputation if you don't cough up the money in the case of defeat.
The steps from section two should be utilized and repeated until you reach about 25,000 gold. Once you hit that mark, two more options become available to you, the neutral auction house and cross-realm transferring. The latter is the single most effective way of making gold, although much planning must be done prior to it. However, the smaller methods you used to hit 25,000 should still be done as regularly as before. Nobody got to the cap with only BoE epics, it takes time and patience!
If there would be a transmog server #1, all transmog vendors would transfer there and the market would get destroyed fairly quick. In general, Roleplay-Servers like Argent Dawn or The Aldor are better server for selling transmog. On high pop servers you have a lot of other players selling stuff and as a result, low prices. On low pop servers prices are acceptable, but you have no buyers. So there is not recommendation for a server really. Its up to you.