The neutral auction house can become one of your best friends. Other players are constantly checking both factions' auction houses to find an appropriate price for the good they demand. Often times, goods are cheaper on the alliance because they have more people, thus, the supply increases as demand decreases and price goes down. If you are horde, feel free to transfer gold to your alliance toon through the neutral auction house in order to buy them item and bring it back to the horde side where it is more expensive. The 15%, as of patch 3.3.3 (down from 30%) is well worth the extra 50% or more you can easily make on one item alone. Blizzard decreased the neutral cut for a reason to 15%, that reason is to get trade facilitated between the horde and alliance. Make sure you take advantage of this! Tips when trading between factions:
First of all, the value of transmog is decided by rarity and how they look. Some items like Minenarbeithut der Tiefen or Pendel der Verdammnis (they can drop in Uldaman) are worth millions of gold. They are worth so much, because they have a really small chance to drop. Other items are bought well because they look good. Best example in this case is the Glorious set, its legplates Glorreichen Beinplatte reach a 100.000 Gold value easily.
Finding the best route for you can be a bit tricky because of the size of the dungeon. I will usually only do about half the dungeon, killing as many mobs as I can in 6 minutes. After 5 minutes, make a group in the group finder and leave it. That will leave you a minute to complete your 6 minutes and you will be ported out on the 6 minute mark, ready to reset and port right back in.
Note that these general principles do not apply to blue or purple items. If you are a first-time player, there is absolutely no reason to purchase these items. None. Sub-80 rare and epic items are only for the alts or twinks of established players that have money to waste (because that's what it is) by showing off their Staff of Jordan (or whatever). Blues and purples are completely cost-ineffective for first-time characters. Within a few levels, you will find green gear that is roughly comparable, or you'll get better blues from instance runs at the same level. So, do not buy these items, under any circumstances, if you are a first-time character (no matter how cool they look). And even if you're reasonably well-off financially, think twice. This is especially true as your character gets closer to levels 58 and 68. Even the most basic quest-reward gear in Outland will have substantially better stats than anything you can buy off the AH for a level 56 character. The same is true of Northrend gear at level 68 vis-a-vis the stuff you'll get in Outland at level 68. As such, smart players stop making AH gear purchases by about level 54 or so, and then just gut it out until level 58. The same is true at levels 64-68. Within the first several quests in Hellfire and/or Northrend you'll have replaced half of your gear in any case, guaranteed.
Swim Speed Potions (used especially at lvl 37 for Faldir's Cove in Arathi Highlands and lvl 57 in WPL). Try to get multiple stacks of these. I also recommend keeping a stack of these always in your bags. You will frequently pass rivers and lakes and can use these potions to pass them faster through out the guide. Please note that these potions do not stack with Druid's Aquatic Form and Glyph of Aquatic Form.
Just before Battle for Azeroth goes live, you will want to log all of your characters out in the zone of Silithus so that you can acquire the quest to talk to Magni and obtain your Heart of Azeroth (refer to our Heart of Azeroth guide for more information about this item). This is the new artifact-type of piece of equipment that you will use for all of the Battle for Azeroth expansion. Once you acquire your permanent neck slot piece of equipment, get back to Stormwind or Orgrimmar to begin your scenario that leads into the introductory areas of Kul Tiras and Zandalar.