First, nobody is forced to buy anything. You can level every class, be it tank, dps, healer, and even every single crafting and gathering class all on one character. Nothing is preventing anyone from procuring the materials they need via their own means.
Selling goods in stacks is by no means market control. Severely undercutting every other person competing with your wares to the point where not only is it not profitable to craft anymore, but the finished (HQ obviously) items sell for less than what the mats cost in total, and then keeping those niches in the market completely locked away from other suppliers by devoting 1 retainer's entire inventory for each item they're controlling and effectively removing sometimes vast portions of content for crafters and gatherers. That's controlling a market. I remember on Balmung with rakshasa gear, 2 characters controlled every single piece, all 78(?) unique rakshasa weapons, armor and accessories, always having 10-20 listed of each at any given time and well below what it cost to craft them, and that went on for months. I personally probably placed 6 or 7 GM calls on them because that directly affects every single other player that would have otherwise liked to craft all or some of those items for some profit, which is a crafter's core content in the game. That's controlling a market in a free market system.
Comparing people who sell stuff in stacks to something as destabilizing as the former is ridiculous. In the real world it's very common to "save more when you spend more!" with vendors selling bulk quantities for a discounted price on everything from soft drinks to illicit drugs and everything in between. Also, admittedly I've been there too, looking for a single item I needed yet having to buy a stack of 50 or something instead out of my own laziness for not wanting to gathering it myself. When you go those 50, or 98 extra items just to get that single 1 you needed, it's not like you don't have a whole stack left over, which, surprise, you can put back on the markets and even sell it in smaller stacks for a slightly higher price if you so choose. So you're not really losing anything and could potentially gain from it even.
Apologies for the long winded run-on sentence but this line of reasoning, in my opinion, is just absurd. This is how open markets work. If I spend an hour crafting a stack of food for example, I don't want to sell them 1 at a time and I think you'd be hard-pressed to find very many others that would want to either. You're also always free to hire a gatherer or crafters via PF or your friends list or FC or what have you to get you some mats or something and negotiating a price with them. Many are willing and happy to make some quick gil doing something they already like. You have access to everything in the game just the same as everyone else does, unless for some reason you don't have expansion installed. No one is forced to do anything but people can be forced out of doing something they enjoy when others actually control markets like those I mentioned. There's a huge difference.
/end rant
