This is not to say that you should never buy gear. Having equipment that is reasonably current while leveling allows you to kill enemies faster, and die less often in the process. Faster leveling = sooner to higher levels (where the real money is to be made). Likewise, death = loss of time. And, as we all know, "Time is money, friend!" So, players should not hesitate to make well-considered equipment acquisitions during their leveling up, but only if they represent a substantial improvement over their existing equipment and if the price is right. Try to find good deals. It should go without saying that you should never purchase any equipment from vendors; always use the Auction House. Look several levels above and below your own for bargains. Don't buy items that you won't hold onto for at least 4-5 levels. And don't always use the buyout option at the AH. Some of the best deals come from bidding and being patient - it'll be two days at the most. Of course, if you play too hard you may have surpassed that weapon by the time you win it. In which case, you may want to auction it again. If you know there's an item that would be great for you, say, five levels from now, keep your eye out for it and bid on it, several times if need be. When you get it, stuff it away for later. That's why you have a bank alt.
Enchants. I honestly wasn't 100% certain which enchants were best, since scaling has screwed up a lot of things. IMO, just pick the enchant for each armor slot that has the highest bonus without level restrictions. The exception is that you'll definitely want Mark of the Hidden Satyr for your neck piece, and probably Elemental Force for your weapon. Both of these proc consistently and strongly across all levels.
The main tool for any serious gold maker is TradeSkillMaster. It is an addon suite with a desktop application that downloads price data and allows you to create rules for easy crafting, mailing, shopping and posting your items on the auction house. It has a premium service that includes e-mail notifications for great deals and importing great deals as a shopping scan.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide.
If i start on a new realms, I am farming a lot of transmog from classic and Burning Crusade Dungeons and Raids. In the beginning, i start to post them if they have at least a DBRegionMarketAvg (Average value of an item on all EU realms of the last 14 days) of 500 Gold. All the other items get vendored. The time you hit a stock of 1000 Items, my value treshold raises to around 2.500 Gold. All the items below that value get vendored aswell. Newly farmed items that are above these treshold will be listed. Why? If I start over somewhere, I want to get a certain amount of gold quickly. Cheap items are great for that. Later on, to save time, Quality is better than quantity. As mentioned above, re-posting the items on the auction house takes quite a bit of time. At this point, I can recommend you the TradeSkillMaster AddOn, it makes your auction house work really a hell of a lot easier.