Got that new ride? Sweet. The gold still sitting in your bank is doing nothing for you.
You can use gold for quite a few things. Some of those things advance your character, either in power, prestige, or vanity. Whatever you feel is worth spending gold on is fine. You should do whatever makes you happy with your gold. But if you're looking to make more gold, you're not going to do it by just sitting on your stockpile of gold. It doesn't matter if you have 1,000g or 1,000,000g.
In the real world, if you put your money in a bank, you generally earn interest. The interest rates on CDs and savings accounts are currently abysmally low (talking less than 1%). But you still do earn money on them. No such thing exists for WoW. Your gold in your bank doesn't earn interest. It doesn't do much of anything, other than being there when you want to spend it.
Another concept that's important is inflation. 100g in Vanilla could buy a lot more compared to 100g in MoP. It was also much harder to come by that 100g in Vanilla. You'd have to go out and farm herbs or ore or cloth or even gold itself to get it. In MoP, do 3 or 4 daily quests, and you're done.
All of this to say: You have to spend money to make money. The more you have on the AH, or available to sell (immediately, or when the time is right), the more you're going to make. Exactly how much of your gold you should have tied up in items to sell is hard to put an exact figure on. If you're gold capped (or more), you may have a hard time having more than 20 or 30% of your wealth in items (maybe not, depends on your server). If you have significantly less, you'll likely be able to have the majority of your wealth in items.
A few things to keep in mind:
- Always have enough gold (or materials) around to fund whatever your main objectives outside gold making are. If you're a raider, that means always having flasks, potions and possibly buff food. Always have money for repairs.
- Do your research before you buy anything. The Undermine Journal is always a good resource, but there are many more.
- Take risks. The greater (educated) risk you take, the greater rewards you are likely to reap.
- Don't spend more than you're willing to lose. When you take risks (which you need to be doing), you're going to lose occasionally. Maybe one day I'll tell the story about my massive Runecloth stockpile. But I digress.
- If you see an opportunity, take it. Even if it's a stretch.