I also watch the market to catch good deals, like so:
And after a while you get a feel for what is "normal" for the market. (BTW those are ship 4 prices).
What's being described above definitely looks like some kind of automated bot listing (we all know that kind of bots exist).
As to the quantity...I don't really know. But if I'm able to buy up that kind of amount for those coveted items, I ain't going to be undercutting people. I will sit there with my high price and sooner or later people will have to buy from me.
I can imagine people with 20 billion meseta buying out everything else and get like 80-90 of an item that is going for 10-20 million. But the undercutting just doesn't make sense. And that points to the items being obtained in a way that has negligible cost; i.e. stolen or dupped.
Of course, there's always the possibility that Elon Musk just decided to troll everyone and bought $1 million worth of AC Scratches. So only SEGA would know.
What would help towards making players feel better, is if a GM could promise that they will look into this behind the scenes to make sure all that stock came about legit, and give us a simple update such as: "We've investigated and found no wrong-doing." Or: "We've investigated and have found dishonest acts, and the perpetrator has been dealt with according to the Terms and Conditions of this game."