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There are two ways to farm for gold in World of Warcraft: farm mobs that drop a lot of cash, or who drop gray vendor items that sell well, or farm for items to sell on the auction house. You should choose which to do depending on how quickly you want your gold, and how patient you are.
If you want to farm mobs for quick money and vendor trash, keep in mind that humanoids are the ones who usually drop coins. The higher level a humanoid mob is, the more coin they usually have. The best mobs to farm in the end will be the ones that not everyone is trying to kill.
For example, there are Nascent Val'Kyr in the Stormpeaks that drop no cash, but will drop Dissolved Soul Essence worth 65 silver. They're also the only place to find Tears of Bitter Anguish, and they're part of a daily quest. So, they're often farmed. While they'd be good for getting cash at 80, you might want to find somewhere less populated.
If you do not feel like trying to farm up gold by grinding mobs, try farming items instead. If you're not sure what to farm, I suggest running to the Auction House and looking up what sells. As a short list, green equipment and weapons, materials from gathering professions, potions, cooking supplies, cut gems, and cloth are usually good sellers. People skilling up professions usually need these items.
Now, decide which ones you can provide, and go find them. If you're unsure where to get some of these items, Google is a good bet to search for information on them. Go gather them. Set a time limit, nothing takes the fun out of WoW faster than spending a whole day farming up copper ore, eternal earth, or mageweave cloth.
Once you have some items to sell, head to the auction house and start listing your items. Before you post your auctions, search for what you're auctioning and make sure you're aware of the price. There's nothing worse that listing an item way too high or low. Too high and it will not sell. Too low and you just lost gold. Now, one or two sales priced low may not hurt you, but imagine if you posted 100 auctions in a month (not hard to do) and lost 5g on each auction? That's a lot of gold.