Preface - I haven't played WoW for any length of time since quitting at the end of WotLK. I dabbled in the next few expansions, where it seemed gold was overabundant, not sure if that's what killed the gold farmers/gold bots/sellers, but I didn't seem to see any, or even think I might have a use for them. However, with WoW Classic, I can see a market for them to come back. Did Blizz ever put something in the game to eliminate this after WotLK, or was it the gold abundance that destroyed their market. If not, I see the gold sellers and websites coming back.
One may think that the AH may suffer a reduction of competitivity, leading to higher prices, but in an healthy server the gold farmer's farm will be taken over by actual players. Giving the chance to a more fair gameplay where you can go out and farm the materials that you need, instead to see them always farmed by gold sellers that sell them to you, without letting you many room to make gold in a fair way.
Leatherworking has three specializations: Elemental, Tribal and Dragonscale. I’d pick up Tribal one. It allows to craft Devilsaur set items. Hunters, Rogues, Warriors, Feral Druids, even Enhancement Shamans and Retribution Paladins are among your potential clients. On any realm there is Un’Goro Mafia on Horde and Alliance, they control whole Un’Goro Crater zone and farm Devilsaurs 24/7. They are making thousands of gold. With Dragonscale Spec you are able to craft Black Dragon Mail set. Good demand as well among DPS classes Warriors, Hunters, Shamans, Paladins, for those who wants Bis Pre-Raid items fast and easy.