I think it is possible for a person to control the market, but the main deterrent is that there is no point to keep on accumulating gil for most players. Goldsellers and people whose primary source of income is flipping (buying at low price, jack up price, maybe corner market) are the only people who really benefit from controlling the market. Once the market has been cornered enough, people usually start gathering/running content to to undercut the people trying to corner the market. Admittedly, a persistent player could keep on cornering the market despite this and make a lot of gil, but again there is little real reason to keep on accumulating gil past a certain point.

I don't think the solution is to remove specialist requirements though. By removing specialist requirements, it actually becomes easier for a person to control the market with just one character, since you no longer need to invest time and money into a second character. Someone who crafts a lot can make friends with other crafters to obtain specialist ingredients at reasonable prices, thereby bypassing people trying to corner the market.

IMO, cornering the market by just gathering and crafting a lot should be allowed, unless it is done by a goldseller, in which case please ban. But cornering the market by flipping is just bad, it makes it harder for people to obtain goods, and those flippers gain gil without contributing anything. I believe flippers should be punished, either with suspension, ban, or suspension of some activities like access to marketboard.