Now that you have the addons and the websites needed for success, you are ready to make auction toons. You will want two auction toons, one on the same faction as your main toon and one on the opposite faction. Do not use your main toon to make the auctions that get you gold! This is one of the greatest mistakes you can make because after a while, people will get annoyed with your prices and selling quantities and begin to blacklist you from groups and some will even boycott your products. Do not risk this happening to you, but rather, create an orc for horde and human for alliance and get them outside the nearest auction houses in Orgrimmar and Stormwind respectively. (As a personal preference, I tend to avoid names with bank, mart, or sell in the name because when negotiating outside of the auction house, you seem less personal). All the addons we downloaded should be enabled for these characters as well.
As others have said, there are free alternatives... which is important to consider if the price of the Zygor Guides are a concern. If speed and convenience are more important, then they're worth it. Zygor will take you through the quests in the most efficient way possible, and their customer support and the speed with which they update the guides when changes are made to the game is impressive. I know I went from 80-85 a heck of a lot faster (in /played time) than my co-worker who doesn't have the guides. And we had a bet going to see who would make it first. For me, it was worth it. Your mileage may vary...
Also get the Heirloom Mount. Being able to have basic riding speed from the beginning will save you a lot of time if you're starting from level 1. Allied races start at 20, so this is a non-issue for them. It's important to note that Pandaren can not use this mount until they finish their starting zone and pick a faction. Being Horde or Alliance is a requirement of the mount.
AQ20 is a really nice transmog farm. The worms called Silicate Feeder are acutally the mobs with the best loot in the raids so be sure to loot them. They are found between the 3rd and 4th boss. Be sure not to kill ANY of the mobs in the room with the 2nd boss as that will start the encounter for the boss. You can go through the whole instance without killing any bosses, so you can choose to run the entire raid or exit after killing the worms by creating a group in the group finder and leaving it.
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Many players who already have higher level characters create alts that they level to a certain point and then stop. Often, these twinks are level 18-19, 28-29, 38-39, etc. for the purpose of going to battlegrounds at the top of their tiers and kicking butt. Since these twinked characters are owned by higher level players with lots of cash, they usually outfit them with the best gear available at their level. Thus, items that require level 17-19, level 27-29, or any other items around this level, with good stats or dps, often sell for much higher prices than they normally would. This is especially true on an older server, and also especially true of blue (rare) items. In general, "good stats" include Cloth "of the Eagle" (for mages, warlocks), Leather "of the Monkey" (for hunters and rogues), and Mail "of the Bear" (for warriors/paladins)as well as weapons with these suffixes that can be used by the right class.