A zygor guide is absolutely not a reliable investment, it's not going to be resaleable so you're never going to make your money back. As a purchase, at this point in the game if you need a leveling guide you shouldn't be playing the game. Leveling up is so fast, quests are easy to understand and chained, and the game puts a mark on your map for where to go for around 90% of quests. If you can't manage to follow in-game quests (possibly with a free add-on), what do you plan to do when you're done leveling? Raiding and PVPing don't just happen for you automatically.
After completing your island expedition and second foothold, head back out to your third and final zone to quest in. Note that any legendaries you were using to level have become inactive and are merely stat sticks now. Feel free to replace them with quest gear as you obtain it. Before heading out, feel free to send your three champions out on new missions!
Be aware of the seasonality of items. Check what items are used in seasonal achievements, such as Delicious Chocolate Cake or Small Egg during Children's Week. When the Darkmoon Faire is in season, Darkmoon cards and decks (Furies, Elementals, Lunacy, etc.) tend to sell well, but prices also tend to get depressed. When the Faire leaves, prices return to normal, but sales volume decreases. The same is true of things like Snowman kits, Red Holiday wear, etc. Holding onto that Snowman kit for a few months, and then listing it in July, can net you a significant profit.
Zandalari troll (8.1.5) - "The Zandalari are a proud people whose origins date back to the early days of Azeroth's history. Their fierce warriors ride dinosaurs into battle, and they possess one of the most formidable naval fleets in the known world. But enemy attacks on their borders—as well as unrest within the king's council—have threatened the kingdom's foundations. By returning stability to Zandalar, the heroes of the Horde can gain a powerful new ally.."