You could and it does make it even simpler (one button does it all) but I recommend against it. You would have to trust the undercut value. Because it also does a scan for the item when you alt click you can see that quite often there will be someone artificially lowering the item price in hopes to drop the price and buy it cheaper. Say some herb usually sells in stacks of 20 but the price for a single unit is 40g, which can go unnoticed (if no add-ons you'll only see the full price of the stack, not by unit) . Someone can easily pick one unit of that herb and post it at 20g or less.
Endlessly queuing the same dungeons over and over again can very quickly turn you off from the leveling experience. This can be counterproductive to your desire to reach 90 as quickly as possible, if you need to take extended breaks to cope or if you stop leveling altogether. God forbid, you may also make ridiculous threads on the WoW forums QQing about the lack of variety in the leveling experience.
Anyway, there are no more shortcuts. There are no more "Tricks". It's just straight questing now since Blizzard nerfed other methods of leveling. You can still do dungeon spam, it just won't be as fast. You can still do BGs, it just won't be as fast. Just about the only alternative way to level that rivals questing is Pet Battles(which makes ZERO sense).
Also note that power leveling is sometimes used to refer to paying someone else to level your character to cap as fast as possible (i.e. they power level for you); a distinction used by Blizzard is to refer to the paid leveling as "power leveling service." This behavior is against Blizzard's Terms of Service. Power leveling yourself, which this article covers, is perfectly acceptable.
Before you begin, you must decide on whether or not to enable War Mode. War Mode makes it so that you will only see other players with War Mode enabled, which also enables PvP activity. Note that if you are in a party with someone not on War Mode while you are, you will be unable to see him or her. War Mode provides an experience buff as well as the enabling of three PvP talents that will greatly increase your power. Regardless of whether or not you enable War Mode, your objectives will remain the same in that you must complete all quest content to unlock flying later on and many of your War Campaign and Allied Race requirements. War Mode enables 10% bonus experience, 10% better rewards from World Quests, and allows for the gathering of Conquest points which can be used to buy gear. If you do play on War Mode, it is recommended to play with friends. Leveling without War Mode will not be significantly more time consuming.
The downside is that War Mode will make you vulnerable to other players, which means you might be killed more often. Whether it’s worthwhile will depend on the zones you’re visiting and how active the other faction is on your server cluster. Heavily populated realms usually mean more players on both sides, and more conflict, but on quieter realms, you may rarely run across an enemy.
The World of Warcraft API has a list of functions that you can interact with and to talk with WoW using Lua. These are your building blocks for manipulating WoW windows and chat boxes and so forth. Looking at these for the first time can be very daunting. But don't be discouraged. After getting more familiar with Lua the WoW API gets much easier, and the documentation gets much easier to read. Keep in mind there are many different ways to accomplish your goals, so when going though the API keep looking until you find a way that best suits what you need.