Many players who already have higher level characters create alts that they level to a certain point and then stop. Often, these twinks are level 18-19, 28-29, 38-39, etc. for the purpose of going to battlegrounds at the top of their tiers and kicking butt. Since these twinked characters are owned by higher level players with lots of cash, they usually outfit them with the best gear available at their level. Thus, items that require level 17-19, level 27-29, or any other items around this level, with good stats or dps, often sell for much higher prices than they normally would. This is especially true on an older server, and also especially true of blue (rare) items. In general, "good stats" include Cloth "of the Eagle" (for mages, warlocks), Leather "of the Monkey" (for hunters and rogues), and Mail "of the Bear" (for warriors/paladins)as well as weapons with these suffixes that can be used by the right class.
Stormheim is the zone you want to start it. For some reason Legion treasures are not worth any experience like they are in WoD...except in Stormheim. I haven't been able to find out any reason for this, and it's possible it may get fixed/nerfed at a later date. But for the time being you REALLY want to jump from treasure to treasure to augment your quest experience.
It surprises me how this farm was not made public until Legion.  It literally slipped through our sights all throughout WOD, where admittedly it would of been a GREAT replacement for the infinite embersilk spot that was quickly nerfed.  Anyway, this volatile fire spot takes place in twilight highlands in the humboldt conflagration area – which is a small fire bed with about 20-30 elementals roaming around.  Each time you kill you have a chance to recieve volatile fire which is worth 50 – 100g per drop.  However, according to my test, the volatile fire has a higher drop chance than the volatile water.  I believe this has to do with the popularity of the volatile water farm in WOD, it's popularity may have of gotten it stealth nerfed.  Sad to say, but this spot is slightly better in my opinion.  REMEMBER to use a potion of treasure finding.
Some players even turn this into their profession by systematically "plundering" vendors in the game world and then selling the items on the auction house at a significant markup. The reason why this works (even for items which are on unlimited supply at vendors) is, that many players don't want to spend time traveling to specific vendors to get hold of a recipe or skill book. They would rather pay a slightly higher price at their local auction house. In some sense, they use the auction house as a "super market" or "convenience store". So it is completely reasonable and legitimate to be the supplier for this convenience store and make money out of it.
For this farm we're taking it all the way back to eastern kingdoms.  We're going to be killing the purespring elementals in the purespring cavern for the item essence of water.  Essence of water is a semi-rare drop that goes for a shocking 252g per piece.  The caveat is that it doesn't drop all too often, but is still worth the trouble.  I wasn't getting TOO many of these, but I was still making bank with them.  The essences are used to create the old-world illusions, so they do have some inherent value.  Simply patrol the area and kill as many elementals as your heart desires.  It's a really cool and fun farm that can be done on autopilot.  Usually there is noone patrolling the spot.
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