Sigh.
The problem has nothing to do with the buying and selling (outside the game) of gold. It has to do with inherent inflationary problems within a game's economy. Buying and selling gold is simply the most visible (and, hence, the most demonized) consequence of those problems.
To address the specific example above - it matters very little if one person (or even ten) purchase a million gp on E-bay. Sure, they might be able to beat any price in an auction, and they might temporarily inflate prices on those items they bid on. But no-one else will have that kind of cash on hand, so if the sellers keep their prices at a level only Mr. Million GP can afford, they'll quickly find that they aren't selling anything. And then prices drop. Simple supply and demand - price something outside of the range of what your demand demographic can afford, and you won't sell it.
Now, the problem comes from this : suppose there are other people who can afford those high prices - say, secondary characters for high level people, or farmers, or bots, or whatever - enough of them that sellers can still move their goods reasonably well at that price range. Then the prices stay high. And it has nothing to do with the guy that bought the million gp. What it has to do with are two separate (but linked) problems that occur in virtually every online economy.
The first is simple inflation. Game economies, by their nature, are not zero sum (nor, for that matter, are real-life economies, but to a far lesser extent). Money is created (every time you kill a mob who drops something, or complete a quest for a rewards) and it is destroyed (via certain expenses, such as consumables, or item repair, or whatnot). In every game yet written, more money is created than is destroyed. In fact, it has to be - if it were not, then resources would get continually more scarce as new players and new characters entered the game. However, it is not even true that the money supply is help constant on a per character basis (and I'm not even sure it would help if it were). So, we have inflation, which means that the longer a game runs, the more everything costs - and there is nothing anyone can do about it either way.
The second problem has to do with level-based games (and, consequently, level-based economies). In virtually every online game (GW may actually be an exception to this, but only time will tell), prices for equipment are tiered based on the level of character for which they are designed. Level 1 equipment costs less than level 20 equipment or level 60 equipment. The cost progression, much like the experience progression, is usually exponential. Likewise, the resource gathering capabilities increase exponentially (but usually on a slightly less steep curve) as level progresses.
This is an acceptable situation so long as there is relatively little economic interaction between levels. Treat each region of levels as its own mini-economy, and things work as they are supposed to. However, in practice that never happens, and there is always leakage of resources from high level characters down to low level characters, which means that there are always a significant number of low level characters who have substantially more resources at their disposal than they could be expected to acquire on their own. And it is this that causes the most problems for an economy (because it means new players are competing with high level characters for their share of low level resources).
Here, of course, is where farming, botting, and buying/selling gold cause their problems, because the end effect is to always (by one mechanism or another) provide characters with more resources than they could be expected to acquire at their level on their own. However, equally guilty (and, in my experience, far more common) are people who simply transfer gold from a high-level account to a second one when they re-roll, or guilds which provide resources to their members, or any other similar activity.
And, again, there is no way to stop this problem without totally isolating those mini-economies (which would be difficult, and probably game-breaking) or (equally game-breaking) somehow allowing new characters to compete on even ground economically with maxed-out characters. Even deleting/banning/shooting every botter/farmer/gold-seller who set foot into the game wouldn't solve the issue (and I doubt it would even make a dent).
Even after that issue is dealt with, there's still good old inflation gumming up the works. And, to be honest, this second issue isn't really an issue - it's just the mechanism by which inflation (represented primarily in the resource-gathering ability of high-level characters) causes problems; fix it, and inflation will find another route.