I'm aware of those that actively play the game to trade etc but duping is something that occurs in every game and I know how to spot it after 20+years gaming and trading in many MMO's myself. The way they list, what they list and when they list are all telltale signs of duping.
I have written tickets, many times. That's the whole point of this post - they don't seem to care about it and it's still happening on the same characters and progressively becoming more frequent.
Thank you for your idea of writing a message though, that is a helpful suggestion indeed.