That's simply how a free market works. It has been this way since server release. Person owns the eco, buys out end game items, suddenly there's none in the trading post so players that didn't want to sell their gear/older players start dumping more, then players start pvming to get these items since they're more expensive now, and eventually the eco holder stops buying and the price corrects itself. Tldr supply and demand.
I was an eco holder in like 2017 and I remember buying out bandos/prims/ags/zenyte jewellery, the price would double for a few days and then I'd check the TP again and there was like 30 of each item again.
Every time this has happened it's been temporary, we're not going to start pming these eco holders to stop spending their cash.
And to be honest, I like when the prices go up every year or 2. I had bulk amounts of random gear that I didn't really sell because the price was too low until now. Selling Acb's and prims for like 25m each felt kinda shit so most of the time I just kept them.