Report an user

Hello, a person promised me to trade his/her galar Zapdos for two of my legendaries (Rayquaza and Giratina). I already traded him/her my Rayquaza for a insignificant, little pokemon and after that he/she was gone. I'm trying to reach out to him/her for days, but the person ignores me. I think that's really unfair and isn't like a good trade should be! Can you maybe help me? Thanks a lot! I sent some screenshot for evidence too.