There are few things you NEED to buy off the auction house to improve your character.

 

Look at it this way: There's virtually nothing you can buy off the Auction House that you won't get an upgrade for through normal play. Every gold piece you spend on a BoE weapon or armor piece is a gold piece not saved toward truly good purchases.

 

In BC, it wasn't really worth it for most classes to buy BOEs when first hitting the level cap. With Wrath of the Lich King, there are a few pieces worth investing in for a fresh level 80 character, depending on the class. Most of these are craftable, so you can usually save money by farming the materials or buying the materials and having a friend or guild member assemble them. Even if you have to advertise in /trade for a combine, you're gong to save money over simply buying crafted items in the Auction House. Know your class, and know what items are worth buying for it on hitting level 80.

 

In Wrath of the Lich King, you really don't need to spend any money while leveling a character. You may want to spend a minimal amount of money every ten levels or so if you're skipping dungeons to power level, and your quest reward gear is out of date, but this should be kept to a minimum. From level 68-80, the Northrend quest rewards should be sufficient.

 

If you're trying to save money, it's for those big ticket items, a flying mount, then an epic flying mount, those higher quality items mentioned above, materials to power level a crafting profession that will pay off with good perks.

 

It's also OK to buy item enhancements and enchanting materials for your gear once you do start getting epics. Be sure to only buy the raw materials, and have someone else do the assembly from your guild or friends list. WoW is a social game, and among the benefits of socializing are the economic. (Just make sure that you give back as well! Whether by trading services for tradeskilling or just helping people with a quest or instance.)

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