I hate to break it to you, but even if he did change security questions or security codes, emails are still recoverable. All major email providers provide methods of recovering email accounts regardless of how deep of shit you get into.
You should contact ROTMG staff and try to get your account recovered. I know how you feel losing something and trying to honestly do business, it happens, plenty of the people on this site get scammed. That's the risk of online trading, even real life trading has the same issues.
1) The #1 issue is that you traded with a scammer. Regardless who messes up, his main objective is to steal things, one way or another, he probably had multiple methods to ensure you never got what you traded
2) The middleman service is a 100% voluntary group, done with no compensation
3) Middlemen don't make it 100% impossible to scam, but do their best to make trades safe
The kid who scammed you has scammed before:
He lives near Detroit, and goes to Oakland Schools
Wouldn't be hard to dox him and ruin his life if you really desire.
No compensation will be provided, sorry.