Selling gold for cash was, for a long time, against the game’s rules. But that didn’t stop large, sophisticated operations from springing up to take advantage of the opportunity to make real-world cash. The outfits that sold gold to players started out by paying foreign workers very low wages to endlessly grind the game, but they later started procuring their gold through quicker and more disruptive methods.
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.
Professions have changed dramatically in Legion. While before you simply had to pay for recipes when you were in town, now the trainers as you to gather certain materials or defeat certain targets in order to unlock additional recipes. How can you profit from this new system? Well, to compensate players with the harder recipe unlocks, the developers have implemented a recipe rank system.