I've seen a number of wild accusations thrown around in the Trading Post, many of which were the result of simple misunderstandings. I would tread carefully. Personally, if I were in a trade situation so shady that the game admins were involved, sure, I'd consider posting a warning here if helpful. But If I were in some kind of private dispute with someone, I would probably keep comments in Trash Talk.
Oohh good idea. Okay okay... I got one. NEVER buy "grass" from someone called "The Easter Bunny". That sumbitch will rip you off!