Multiplayer arrived on March 9, 2023 with the release of patch 0.9. So while players may have been playing Last Epoch for years now they’ve only been playing online for a short time, in comparison.
While I’m sure the economy of Legacy will be a little wild at first there are certain measures that should help stabilize it, such as items can only be sold once.
Also, the Favor costs associated with trade will likely to be the biggest limiting factor on trade. That dev blog I linked in my initial post highlights all of this information so it’s certainly worth a read, especially the expandable FAQ. That said, its a new system that’ll likely undergo additional alterations and balancing over the course of the games lifecycle.