Steal materials in traid (Who will I contact?)

Hello.
I have a problem, the fraudster stole my materials during the exchange.
I have screenshots but I dont know where to write.
I ask your advice.
Thank you for attention.

Screenshots:
https://drive.google.com/open?id=0B8...jNJdWE0UzFjemc
https://drive.google.com/open?id=0B8...UFHSUM5MmMtTmM
https://drive.google.com/open?id=0B8...kJzRy1UdWhQMm8

Just use the in-game report button to GM.