Greetings
Well a pretty easy trick, atm the quest you get after completing a Darkmooncardset is not up to date. The Quest says that the Darkmoon Guys only visit Goldshire and Mulgore, Shattrath is not mentioned. So you can easily write a ticket claiming that you just missed the Shattrath Darkmoom cause of the wrong questdescription, the GM will give you your Trinket.
I know, my english sucks but I give my best
Also, I personally think that this is not a normal GM Scam because there IS a reason why the GM HAVE to give you your treasure (trinket). Btw it worked twice (got my Crusader Trinket and a guildy got that Pala-Tank-Trinket)