The Garrison Trading Post opens quite a bunch of gold making possibilities. Beginning with level 1, this building gives you access to a merchant that sells crafting materials for Garrison Resources (GR). Of course, as you level up your character, you will be needing as many Garrison Resources as possible, to level up buildings, and your whole Garrison. But once you have all your buildings at level 3, your resources will begin to pile up, as you complete follower missions.
This is where your Trading Post comes in. So, to make some gold, all you need to do is trade in some of your Garrison Resources surplus, for crafting materials. Needless to say, you should exchange your GR surplus for the crafting materials with a current high price, in order to make the highest profits possible. Some of the best crafting materials that you should invest in, are Sea Scorpion Segments (or most fish in general), Rylak Eggs, Blackrock Ore, True Iron Ore, Frostweed or Gorgrond Flytrap.
Another medium building that complements that Trading Post well, is the Lumber Mill. This building allows you to chop down trees and turn them into work orders, for GR. So, with a Lumber Mill, you will have to farm trees in Draenor, get extra resources this way, and trade them at your Trading Post for valuable materials, to sell in AH.
At level 2, the Trading Post opens another feature, an auctioneer robot which functions as any other Auction House NPC in the game. But this robot is broken, and to repair it you must gather various parts which drop in instances, from any mobs in Draenor, or from Ashran.
Now, most of these parts have a pretty low chance to drop. And you cannot farm them only if you have a lvl 2 Trading Post. So, this opens a new possibility to make gold – the farming of these parts, and selling them in AH. Most players give up after farming one or two and would rather buy the rest. Even if you don’t farm these parts specifically, to sell them in AH, it’s a good idea to just have your Trading Post at level 2, so that every time you grind somewhere, you’ll have a chance to get one of these, and score a nice 2-3k of gold in the process.
Evidently, the most expensive parts are the ones that drop from Ashran, because it’s a PvP area. These can go for up to 20-30,000 gold.