Blizzard has rules for their RMT, though, and Zarhym lays a few of them out: they won't charge for any item that means anything in game -- cosmetic items and looks are fair game, but actual gear or "integral services" (whatever that means exactly) is a no for them. They won't charge for anything that was free before, so creating up to 10 characters on a realm, for example, will always come with the subscription (though adding more may eventually be possible with an extra charge). And Blizzard's RMT comes as a game mechanic itself -- they choose to charge for things not just because there's a cost for them, but also to "curb their frequency," to keep all players from doing them all the time.
It's an interesting idea, and it's definitely a lot more player-friendly than charging for things like, say, horse armor. You could also argue, of course, that something like the WoW TCG is also a kind of RMT scheme, since you have to pay real money for real cards to get in-game items (even though Blizzard has made sure those items are cosmetic as well). But paying for transfers and changes is a little sneakier -- Blizzard is slowly wading into RMT, so far successfully dodging all the sharks in the water.