There is a simple to increase your gold amount when it comes to buying gear on the auction house - just don't do it. Seriously, just don't. Leveling is easy and fast as it is. There is no reason for you to waste your money on gear that you will replace within a few hours. I know you feel that you will be able to level up faster if you invest in it and this will help you reach the cap faster and allow you to make more gold, but for the most part, it simply isn't worth it, unless you are rolling in gold already.
Now, if you are rolling in gold, just go for it. There is no point in having allot of gold if you don't use it. Heck, my last two alts have set me back 2-3 thousands each before they got to 80. Just don't do it if you are low already.
Now, I can think of an exception or two for this advice. If you are a spell caster at a low level, got to the auction house and buy one of those low level wands that enchanters make. It will make your leveling much simpler, all the way up to level 20. Those wands do plenty of damage and cost no mana to use, so you will have less downtime and level faster. Just remember that there is a point where you should start using spells exclusively. No one likes a wanding warlock in Uthgarde Keep.
Another exception would be if you are leveling as a tank. The Northrend low 70 tanking set is great and will really help you stay alive, so you should probably invest in it. You probably could make it without it, but you will make the lives of your party members much easier. Also, feel free to fill those gem slots with some blue or even green stamina gems. They might not be as strong as epic, but they will provide a nice health boost.
Blacksmiths should also be aware of the above tip. People buy those tanking sets, so make sure you check if you can turn that fact into profit.