I think the problem is exactely like Eiryl said, almost.
When you have people who can play only 3 hours a day with a single account and people who can play 6+hrs a day with 4+ toons, of course the latter will make an insanely higher amount of gil than the former, and as such he'll be more keen to spend larger amounts of gil (driving the price of stuff up).
But this is a basic mechanism of how things work in the virtual economy of MMO game, I don't see anything special with it. Love it or Hate it, can't do much about it.
What made things even worse, much worse than this scenario I just described, are two separate factors.
1) Large amounts of gil that were generated through hacks and that never got completely (not even remotely) pruned from the economy
2) Large amounts of gil (in the grand scale) that get constantly generated with legit ways through NPC selling stuff (sparks being the main but not the only culprit here)
When you're an average player with a single account that might not seem like a big deal, because let's face it, it's not.
But when you have multiple accounts, lots of accounts, and that's all you do all day long, then it becomes a much serious problem for the community.
This creates an insane amount of inflation and the result is that price get driven up. People selling (legitemately) make more money, which will make them more keen to spend more money on stuff they have to buy, and then sell, and blah blah it's an endless circle.
Prices of HQ items, as rare as they are, should've never touched the >400 mils it's come to these days.
It happens so often we got used to it. This is not a good sign, it's a sign of what I described so far.
If you ask me I don't think SE can do anything to solve this problem. Is way beyond solution, way too late.
They can try to somehow slow it down or contain it maybe, and it's what they've been trying to do with "gilsink" (stuff you buy from NPC for relatively high price, i.e. removing gil from the market flow). One example being the new mats added from guild NPC.
Imho that's nowhere good enough, people in the past over these boards have come up with plenty of interesting gilsink options that would be really helpful to the end of removing gil from the market, but we all know it won't happen, alas.