Congratulations on reaching Level 120! Now that you have hit Level 120, we recommend looking at our Easy mode page and Talents section to learn how to play at max level. To get yourself prepared for the end-game content ahead, we strongly recommend this Battle for Azeroth Raiding Lexicon/Spreadsheet by Medivh. It is a gold trove of information that will help you when gearing up in Battle for Azeroth and more.
Potions serve more utilitarian purposes, and it's up to you to decide whether to make use of them. The most common is the Swiftness Potion, which can provide you a handy boost in speed while going after annoying quest objectives indoors. These are generally MASSIVELY overpriced, because people know how useful they are. I opted not to spend the gold since Warriors are already fairly mobile, but slower classes may benefit greatly from these.
When buying raw materials from the AH, keep an eye out for bargains. Choose only the cheapest items available to keep costs down. Try scanning the whole section of item listings in the AH to check prices; often the prices for stacks of items in the AH are much cheaper than individual pieces (for example, a single Thorium Bar may be priced at 3 , but a stack of 10 may actually be priced at only 2 per Thorium Bar) — you may spend more to acquire your materials, but you also save more and ensure larger profit in the long run. You can find a lot of terrific bargains this way.
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