Discussions of Real Money Transactions (RMT), in which players of some game go outside the game to "buy" in-game items or currency, always seem to provoke someone to offer something like the following notion: "Well, people want to do it, so gold sellers will always exist to satisfy that desire, so it's always going to happen and there's nothing we can do about so we might as well accept it or even do it ourselves."
The fact that we can't prevent 100% of some undesirable thing does not somehow transmute it into a desirable thing. Nor does it imply that 80% or even 60% prevention aren't worth trying for. If the benefits of reducing some problem exceed the costs of enforcement to a useful degree, then enforcement may be worthwhile even if it's not complete. An inability to achieve perfection is a juvenile and utterly unconvincing excuse for accepting bad behavior. And that goes for in-game economies as well as real-world socioeconomic systems because human behavior is at the heart of both.
Speaking just of games, a game that's designed to tell a story within a literary universe (as distinct from our own real universe) means that at the boundary of the gameworld is a "magic circle" within which objects and rules from the real world can be disruptive. Such a gameworld is one that needs to have RMT minimized through design as much as is reasonably possible. Maybe that means soulbinding, or doing away with tradable objects almost entirely. Maybe it means preventing trial accounts from accumulating or trading any meaningful amount of anything. Maybe (probably) it means a combination of all of these and more. As long as those design features are within reason, then a game whose difficulty structure is dependent on people playing the game as the designers envisioned it probably needs to do them.
A game (or social world) that's deliberately designed to be a "velvet rope" or microtransaction-based virtual world, on the other hand, can do as much RMT as it wants. If that gameworld isn't intended by its creators to be a distinct literary universe, then it doesn't have to be one, and people can import whatever bits of the real world they like.
The key thing to see here is that it is correct to expect people to play by the rules of the game according to the folks who created the game. If someone doesn't like the rules of a particular game, then they shouldn't play that game. To play it and deliberately break the rules -- rationalizations for doing so are irrelevant -- is to foul up gameplay for everybody else who is trying to cooperate at playing the game as it was meant to be played.
The only reason developers haven't taken the goldsellers to court is because no one wants to take the chance that some utterly clueless judge will decide that developers don't actually own the ones and zeros of the data in their game.
Which means that if game developers don't like RMT, it's up to them to design their game so as to minimize in-game opportunities and/or rewards for such behavior.