Gold/Game Time Trading
Blizzard have announced that, in an upcoming patch, we will be able to purchase "WoW Tokens" in their in-game shop. These tokens can then be sold on the Auction House for Gold (meaning you can buy gold for cash legally) and used to purchase game time (meaning you can use gold to pay for your subscription).
There has been no confirmation of either the real cash price or the gold price for these tokens yet .. although it has been estimated that the "real" cost of them will probably be around the same price as a month's subscription, which makes sense.
See Blizzard's announcement for more information.

I know Blizzard are doing this as part of the fight against account hacking, but this is another Candy Crush move. Perversely, it should be more than possible for the gold sellers to use this to their advantage.
I don't know what the market will be like on Lightbringer, but for people on realms like Silvermoon, you could rack up a nice chunk of gold if you have a disposable income.
Or save a nice chunk of cash if you have disposable gold :P
Looking at the link, there are some more specifics .. but several other games have introduced something similar:
Q: How do I buy a WoW Token for real money?
A: WoW Tokens will be available for purchase for real money through the World of Warcraft in-game Shop. You can access the Shop through the row of feature buttons next to your character’s bags.
Q: I need gold! How do I sell a WoW Token to another player?
A: You’ll be able to sell WoW Tokens through a dedicated Token exchange in the Auction House, located in a new Game Time section. WoW Tokens cannot be traded or sold any other way.
Q: How much gold will I receive when I sell a WoW Token?
A: The gold value of a Token will be determined dynamically based on supply and demand. When you put a Token up for sale, you’ll be quoted the amount of gold you’ll receive upon a successful sale. If you then decide to place the Token up for sale, that amount is locked in, and the gold will be sent to your mailbox after another player purchases your Token.
Q: I need game time! How do I buy a WoW Token from the Auction House?
A: When you visit the Auction House, you’ll be presented with the current market price for a WoW Token in your game region—there’s no bidding involved, and all Tokens in a game region are priced the same at any given moment. If you decide to purchase one, you’ll receive it in your mailbox, and can then immediately redeem it for game time.
Q: How much game time do I get by redeeming a WoW Token?
A: You’ll receive 30 days of game time when you redeem a WoW Token.
Q: Can I resell a WoW Token after I’ve purchased it for gold?
A: No, each WoW Token can only be sold once. After you purchase a Token for gold, it becomes Soulbound. At that point, it can only be redeemed for game time.
Q: How long should I expect to receive my gold after putting a Token up for sale? How does the game determine whose Token to sell?
A: The amount of time it takes to receive your gold after putting a Token up for sale depends on a variety of factors, including the current supply and demand. When you list a Token, you’ll be quoted an estimated time based on the current Token supply and the rate of recent transactions, but the actual amount of time it takes to sell is likely to vary (note that Tokens do not expire). In most cases, Tokens are sold in the order in which they were put up for sale; however, there are some exceptions to this, such as when a Token purchase is undergoing verification.
Q: If I buy a WoW Token from the Auction House, how long does it take to receive the Token?
A: Just like a standard Auction House purchase, it will normally arrive in your mailbox nearly instantaneously.
Q: If I’m buying a WoW Token for real money from the in-game Shop, how long does it take to receive it?
A: Once you successfully complete a purchase, your Token should arrive in your inventory (or mailbox, if your inventory is full) almost immediately.
Q: Do Tokens expire? What happens if I accidentally delete one?
A: No, Tokens do not expire. They also can’t be deleted, so no need to fear accidentally trashing them.
Q: Can I stop my auction or receive a refund after it was purchased (for gold or real money)?
A: All transactions are considered final and auctions cannot be cancelled, so make sure you’re certain you want to proceed before listing or buying a Token. To help prevent unintentional transactions, you will be prompted to confirm your purchase or sale before it’s finalized.
Q: Is there a deposit? Does the Auction House take a cut of the Token seller’s gold?
A: There’s no deposit, and the Auction House does not take a cut of the gold for WoW Token sales. The standard deposit is designed to dissuade players from spamming the auction house with items that aren’t selling, and the standard cut is designed to dissuade players from buying and reselling items for minuscule markups. These issues don’t apply to the WoW Token, so there’s no need for a deposit or cut.
Q: Will each game region have its own WoW Token exchange? Will everyone in a game region have access to it?
A: Each game region—Americas (including ANZ realms), Europe, Taiwan, Korea, and China—will have its own shared WoW Token exchange. We’ll share additional details at a later date.
I guess really it depends on how much spare gold people have, how much the tokens cost, and how many people want to buy them (for either gold or real money).
I did notice one Q/A mention a possible delay whilst waiting for verification .. perhaps this is part of the process that tries to ensure it isn't exploited?
