Plex was ~8 years ago. But, they had older system GTC going back to I think 2004 or 2006.
The real issue is some people could make that much cash in 30-90 min, but only really needed 1-3 plex per month, which kind of destabilized the system.
EVE has had formalized rules and forums for GTC trading dating back to roughly 2004 or so, at least. There was nothing automated but it was 100% officially sanctioned and scamming in a GTC-for-ISK in the official forum would get you banned.
That actually made it rather unique in EVE, where the official line was "trust no-one" and social engineering was officially encouraged. Actually hacking is over the line, but subterfuge, social engineering, and privilege exploitation have brought down superpower alliances in EVE.
Most famously: BOB. The Mittani sends his regards. (RIP vilerat - your horrible tattoo will never be forgotten)
The real issue is some people could make that much cash in 30-90 min, but only really needed 1-3 plex per month, which kind of destabilized the system.