Guy #3 is the closest to the "problem." And I use the term loosely, flanked by quotation marks, because I don't necessarily believe there is a problem.
People pay what an item is worth to them. If I shop around, I can find steel ore pretty easily at a good price, or can sell my steel ore pretty easily for a good price. But if I want 50 steel ore right now, in bulk, and don't want to dicker around looking for good buys, and I want to get back to the game as quickly as possible, I'll pay more. And if I'm in a hurry to unload a bunch of ore and free up some inventory space, I'll sell cheap just to be rid of it. However, it doesn't matter how many mesos are pumped into the system or what a good piece of equipment is selling for. I will never, ever, get ten million mesos for a piece of steel ore. That hunk of ore just doesn't -do- enough to be worth that much, even if I'm totally loaded and have ten trillion mesos to my name.
But replace my steel ore with a rare item, that there's a snowball's chance in heck that I'll ever get from a drop, and you get something I'll easily pay ten million for, if not more. But even that rare item has a limit. There comes a point where the other stuff I can get for my 20 million mesos is worth more to me than a weapon with 2 attack better than my current weapon. Sure, if I'm level 200 with mesos to spare and nothing worth spending my money on, heck, I might buy that rare scroll or weapon that's a smidge better than what I'm using now for more money than anybody else will be able to pay. Luckily, there aren't very many level 200 trillionaires in that position.
For the most part, prices are limited by what people are willing to pay. Dump ten trillion mesos into the system, and prices might go up a little bit, but only because people are willing to pay them, when they wouldn't have been willing pay them earlier, until they had the extra cash. I won't give you my last ten million for that staff, but I might give you ten million for that staff if I have another ten million to spend on potions and scrolls after paying you.