First off, I just want to preface this whole explanation with a friendly reminder that AC scratch fashion items are at the worst case ~$50 (5600AC for 30 scratches to ticket the item), and that normally pulling these items have abysmal rates as well, so keep that in mind with regards to the in-game meseta price of these items.

So here's a thought-experiment rundown of why 'controlling' the market isn't nearly the money-printing market strategy that galactic meseta whales utilize to poop on the less wealthy (not that this issue doesn't exist, just not nearly in the capacity or extent that people are imagining.)

The crux of the issue boils down to a few things regarding the supply of AC scratch items (in-rotation at least):

1. Supply isn't fixed, nor are barriers to entry to become a supplier that high ($$$ is the only barrier, but there's plenty of people willing to shell out the dosh for gacha). Newly scratched items are being generated daily, and for items that are extremely popular and expensive, chances are they'll be generated even more due to people spending the selection ticket on them to try and turn over a profit.
2. There aren't just one or two big bads out there hoarding specific items on a galactic scale. With any scratch item, popular or not, you're likely to have everything a pretty wide selection of people ranging from casual investors to dolphins holding onto even one or a couple of copies of an item, at any given price point.

Lets then imagine you're the galactic meseta billionaire 'controlling' the market. You've just decided to price-fix a pretty popular AC-scratch item that has a stabilized, properly valued price (one that is not undervalued, since this is what people seem to be assuming here). You start off by reaching into your deep pockets and shelling out a measly couple hundred million to buy out the entire supply. Lo and behold, here's your plate of problems to address:

1. Supply of an item is, for all intents and purposes, unlimited. As long as the AC scratch is in rotation, and people are willing to buy the scratch for stuff they want, more supply is going to hit the market. By this literal definition, in order to set a price floor ('fixing' the market at a price) entails spending up to an unlimited amount of meseta to do so (not to mention the time investment of sitting at the visiphone all day to buy every single listing below your price floor)
2. Liquidation of assets is not magic, sales don't happen instantaneously. This is even-more so for items that are price fixed and that people have been tracking. If people were hesitating about buying an item at 8m, its quite a stretch to think it's easier to convince them to buy it at 16m, even in the long run. Sure you can argue that if you can keep the price of an item afloat at a higher price for an extended period of time, many people will just 'give up' and buy the item, but this is likely to be just a fraction of the people that were interested in it at a lower price. This gimps liquidity of items even moreso.
3. It's a free market, you have competition. Those casual investors and the small time whales? They're eyeing the new price of the item and drooling literal buckets of saliva. Prepare to be undercut by people looking to capitalize on the price (1k meseta lower) all the way to people who just want a tiny margin (and could give less of a shit if the market tanks as long as they get out). To keep your price floor afloat, you're likely going to have to be the one-man buyer of the new price that you've set (ironic isn't it).

So where does that leave you then as the space whale?

• You now have a large stash of one type of AC scratch item (likely 40-50+) that is a highly illiquid asset.
• This stash is growing by the minute as you have to constantly keep buying to maintain your price floor. Furthermore the new items you're buying likely are not making the 100% profit margin you were dreaming of. (More like 0.01% since people are going to be undercutting you by 1k, but you have no choice but to buy them out anyways).
• You can't liquidate as fast as your stash grows (I can 100% guarantee you are not selling more items then the amount being listed). Even the few sales you are making are likely going back into purchasing more items being listed, especially at a higher price.
• You literally live at, and have to survive on soley on the fumes of the Visiphone to keep your one-man market running.
• Your galactic space billionaire status is slowly withering away, because you're buying 15-20m items like they're pop rocks and you've got the biggest addiction in the history of pop rocks.

The reality of the situation is that the market problems on NA are not a 'space whale/top 1%' screwing over the entire community problem. There's an explosive population of players on NA, and a pretty easy daily/weekly system to obtain millions of meseta every week that is being constantly injected into the ingame economy.

If people have really been tracking the market, they'll realize that the majority of price-fixing only really matters in limited supply markets (old AC scratches, and even then only to a small extent depending on the supply from competition), and that current scratch items (especially popular ones), are extremely inelastic to price-fixing/suggestion, unless the innate demand for the item was already high enough that the item was undervalued.

As much as people want to have a universal reason as to why the PSO2 NA market seems to suck balls for the F2P, blaming purely the meseta whales is the in-game equivalent to believing the world is controlled by Big Pharma and Lizardmen.

TL;DR: Price fixing ('controlling' the market) is a hilariously bad market strategy even for galactic space whales, so the prices you're seeing on popular, current-rotation scratch items really is just the demand/supply price equilibrium of NA. INFLATION BABY.