The entire Jade Forst is our farming spot for this one.  Literally, the entire Jade Forest.  The Jade Forest is crawling with porqupines and if you have a mage character, each one of them has chance to drop the polymorph porqupine tome.  Which is excellent, because these tomes have a selling point of 13,000g across the board. They are really easy to get, and noone really pays attention to these spots.  Try it if you like, I was able to get 2 on video, and have already sold one of them.  
Beyond that the constant amounts of RNG on top of RNG has made the game not only frustrating but unfun and unrewarding. It is clear with the current game design decisions the goal of bfa was not to create an exciting experience and to refresh world pvp and faction pride, but to test the player base and see how long subscriptions could be pushed with the least amount of content. Class specific raid gear, tier sets, removal of any sort permanent player progression and a complete draining of class skills has left BFA with less and forced players to grind more only with the knowledge that like Legion all your work at the end of the day will be worth nothing due to end game catch up mechanics and removal of the azerite necklace at the end of the expansion.
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!
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold.  If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.