[Endrance]

MMORPGs are momentaly basically about character progression, hence gaining better stats through leveling and gathering equipment. Because it's more enjoyable to be "strong" in a community than in an offline game (where no one gets ****ed off by yourself taking advantages), many people would pay alot of money for, which I find sad, since you shouldn't be able to buy the main core of strength through money, not even in games. Currency sellers are usually disliked more, as they tend to monopolize viable hunting grounds or profitable crafting recipes from actual players.

In short: Currency sellers and buyers are the cheats of MMORPGs. And simply no one likes to be cheated in a multiplayer game.


EDIT: The solution for this is rather simple though. By making gameplay more important in terms of strength than character progression through impementing player-skill oriented real time action-combat, paying a huge amount of money for minor strength increases would pretty much be meaningless.
Character Progression could still be there like it used to be, just in another way than increasing your chracters strength - such as unlocking cooler and harder missions, way cooler looking (while only little stronger) equipment that maybe unlocks harder to use, but little more efficient moves/combo options (which'd provide meaningful gameplay change) or interesting features (very generalized, but there are many options). So the feeling of progression wouldn't be lost, it'd just be of minor meaning. This did work already very well in several action-gameplay oriented RPGs (the most popular one probably being Monster Hunter from Capcom).