@Albus-Rabbit That's pretty much it. It's a simple rule of supply and demand. If there has been no significant changes in the supply&demand large margin undercuts only have a short term impact on the item price, generally speaking. People who bothered to study PSO2 market at least a little bit can usually make a fairly accurate predictions, you don't even have to be a JP veteran to do this.
Let's say a someone wants to sell an item X for 1m. He goes to the market and finds out the price dropped to 600k, takes a screenshot and posts it alongside with a comment crying about undercutters ruining the market. If the item X was truly worth 1m the person would post the same screenshot but with a very different comment. Something along the lines of "Look at these idiots listing stuff for so cheap. Yoink, my meseta now! Time to relist these for 1m".
To stay on the topic. As someone who has played the JP version, NA always felt like a scummy cash grab to me with all the changes and dumb new additions such as Fresh Finds shop. NGS reveal and cosmetics carrying over pretty much confirmed this.