The next step on the stairway to the gold cap is the internet. Often times, WoW players forget that various websites can have good information. However, many of them are dangerous and just filled with lies or exaggerations. It is my advice that you stick to three main websites: Wowhead.com, Worldofwarcraft.com, and Mmo-champion.com. On wowhead.com, one can see average prices overall on every sellable item currently in the game. This is a nice start; however, most servers have economies that differ greatly. I played on one server where a +healing red gem went for 100g and another where the same gem went for 55g. Swings like this are just the beginning. Random drop epics, especially those BoEs from newer content sometimes differ by thousands of gold! (Consider this fact especially if you are planning on transferring, as this can be the greatest gold mine in WoW). Worldofwarcraft.com provides a little known list containing the most common bought items on the auction house across every server in an aggregate sense! This means that they average every server before listing the results. Unlike wowhead.com, this presents the reader with a much more accurate portrayal of what they may expect on their own given server. Results will be slightly different, but this tool should not be overestimated.
Similarly, when earning by volume, do not overload the AH with your items. Basic supply and demand: too much supply will make your wares too common and unwanted. Furthermore, with great demand and a cheaper price, you may end up starting a price war with your competitors, with them actively trying to undercut and outdo you. Moderate the amount of items you're selling on the AH to small batches, refilling them only when sold out.
Most of the players make the mistake of only listing 30-50 transmog items on the auction house and then hoping to get half of them sold until the next day. Me and a lot of other gold farmers and transmog sellers do have 3.000-10.000 items on the auction house across various realms. It does not mean you have to do it that way – but if you have 1000 items listed, chances are higher you can sell up to 10 per day. With only 100 items listed, you have to be lucky to sell one. Of course, the sells are not guaranteed if you post more items. But i’m sure you have a lot more fun if you open your mailbox and 10-20 items are sold