How to Farm Gold
There are many ways to farm gold in the client but they can be categorized into a few different types. One of which is arbitrage, another is manufacturing, the last is servicing. In some aspects they are a reflection of real markets and not of gold farms, but this is of course tempered by the nature of the internal rules governing trade. This naturally results in certain idiosyncrasies as well as interesting trends and different theories arising on supply and demand patterns. Of course there is also grinding, also known as farming, but that is really self explanatory and understood by most in the market. According to some newspaper reports, it has also become a secondary profession or occupation in some developing countries.
Arbitrage deals with profit making where nothing constructive is done. The profit maker simply performs the task of matching demand with price and availability. Simple reselling is one such form of arbitrage. You buy the goods at their natural lows and keep it till the time is prime for reselling. Easy examples are purchasing battered hilts at 6k - 7k and later pricing it at 8999 and waiting for a sale. Other forms of arbitrage happens during temporary enforced monopolies. Someone could buy out all the primordial saronites and realist them at a mark-up (this is a bad example, it wouldn't be profitable due to the auction cuts, depending on how advanced your server guild progression is). But the concept works. It is similar to buying stocks or forex markets. For further help you can also check out some gold making resources. That said it is a bit of stretch to extend it to real gold like gold bullion but some economic principles are the same.
The other way to obtain goods is through manufacturing. This is also rather profitable with the conjoined use of certain addons, notably little sparky coupled with auctioneer. You purchase mats or materials on the cheap, and then craft them into a form for sale. Common examples are ICC recipes or TOC recipes where you have a substantial profit margin due to rarity. The different professions offer manufacturing profits to different extents at different phases of the game. For example, jewelcrafting is in the doldrums now, but if paired with enchanting, the saronite shuffle is still viable with some time and add-on automation.