WoW is introducing a PLEX-like item: the WoW token. A player can buy it from the cash shop and sell it for in-game gold on the Auction House. The buyer can then use the item and get a month of game time.
As you know, I don't like these schemes. I simply think it is a bad idea to let strong players have a "free ride" at the expense of weaker players. WoW rests on a broad base of relatively equal subscribers, and I think that narrowing that base will end up weakening the game as a whole. As well, I don't think it's a good idea to incentivize your strong players to start gold farming in earnest. In fact, I would almost prefer WoW to sell gold directly, rather than this indirect method.
But I've lost this battle. (Though I rather doubt any game company even noticed I was fighting it.) It's clear that these sorts of items will be standard in MMOs from now on.
Let's take a look at this specific implementation. There are some interesting nuances here.
Unlike previous implementations like PLEX, this item is not liquid. The item can only be sold once. After that it is soulbound and must be used. This means that there are no speculators involved, and the only buyers are people who actually want the game time.
As well, since the item cannot be traded, the potential for scamming is eliminated. As are external RMT-ish resellers and lotteries.
The item can only be sold through a special AH interface. Of particular note is that the player cannot set the price. Instead the game offers the player a value, and when the item sells, the player gets the offered value. In theory, the offer and sale could be different, but overall I imagine the difference will be slight.
This can be modeled as the AH purchasing the token from Player A, and then selling the token to Player B. The price offered to Player A will probably be some sort of rolling average to discourage waiting for specific windows to sell the token. For example, waiting until Saturday to sell the token because that's the day the most people are online.
I'd guess the A-side price would be something like the average price of all tokens sold on the B-side over the last 7 days. Then the B-side price would be some automatic bidding mechanism where the price increases whenever there is a sale, increases if there are no tokens available to sell, and decreases if a period of time passes without a sale.
This process probably also allows Blizzard to set a floor or ceiling on the B-side price if they deem it necessary, most likely to stop side effects from a dupe bug or similar exploit. If the price of the WoW token suddenly triples, that's likely a sign that someone has figured out an exploit.
Blizzard's clear intent is to eliminate all possible customer service issues. The process is entirely automated. There is no point where a player can exercise choice, other than to sell or not sell. So there's no opportunity for a player to make a mistake and hurt themselves.