If you have 40 people in an Internet cafe testing various ways to dupe through disconnects, specific trade interactions, etc. you are bound to hit eventually. Then you utilize that method and sell it to a few other trusted insiders. Eventually someone leaks it by accident or maliciously, and it is patched. But you’re already on the next method.
And dude above is right - as long as JSP and RMT exist, so will dupes.
Just enjoy the ride and buy when it’s cheap, sell when it’s high, exploit early and exploit often as is the Blizzard way.